Please refer to the latest edition of this guide here:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=206042835

• Introduction
  - Short note on Serious Sam and the contention of this guide.
  
• Serious Difficulty
  - What has changed compared to previous difficulties.
  
• Weapons
  - Bullets
  - Shells
  - Projectiles
  - Other
  
• Enemies
  - Statistics on the enemies
  - Health
  - Damage
  - Point Values
  - Tactics
  
• Environment
  - How to make the best of where you are.
  - Indoors
  - Arenas
  - Corridors
  
• Coping Mechanisms (Broad Strategies)
  - How to survive the never-ending onslaught.
  
• Co-op
  - What changes in co-op with additional enemies.
  - What changes in enemy behavior in co-op.
  - How to support your friend.
  - Shortcuts.
  
• Secrets!
  - Easy secret weapons and items for you to smash and grab for your Serious playthrough.

Welcome, to what I may hopefully dub "The Unofficial Serious Sam Serious Guide".

Serious Sam has been renown for several things since its first release in 2001, predominantly its core gameplay of "run'n'gun" and its ability to overwhelm the player with ridiculous circumstances of outnumbering, sometimes literally 1,000:1.
What makes Serious Sam so unique and so difficult comes from its absolute simplicity. The game, as said by the developer themselves, was never meant to be anything more than a lot of enemies and a lot of bullets. What was created was this unique game, unprecedented by anything similar, where the player is given large guns to kill large amounts of anything -- simply for the fun of it.
Yes games like Doom had existed beforehand, but never with such insane amounts of enemies.

What I personally find disheartening is that many players, in fact a majority of all who have played it, are completely inept at facing Serious Sam at it's true difficulty. Most claim that it's far too out of their skill-ceiling, and others claim that it's just simply 'unfair'.
Serious difficulty provides enemies that do more damage, more enemies in terms of quantity, different enemy spawns, as well as double the ammo count!
These are the only significant differences, yet it's like experiencing a whole nother game, and I can't bring myself to play on a difficulty any lower.

As an arrogant, self-proclaimed veteran, who had purchased Serious Sam for $5 at Harvey Norman at the age of 11 and has been playing it ever since, I feel almost obligated to help other fans of the game get the absolute most out of what the Serious Sam games have to offer.

Throughout this guide, I will discuss how to cope (broadly) with the difficulty, not by holding your hand and giving you a walkthrough, but by giving you a jist on "what you should expect when you're least expecting", and how to keep strong against the most Croteam has to offer.

Serious difficulty is, hands down, one of the most renown difficulties of the classic/arcade FPS world. It's renown for its brutality in numbers, both in enemies and damage.
Much like DOOM's "Ultra-Violence" difficulty, Serious Sam's "Serious" difficulty is 'how it should truly be played', as stated by members of the community.

Here is where you can find an in-depth statement of everything you need to know and what exactly is in Serious difficulty[www.serioussam.wikia.com], showing the description, the pros, the cons, the multipliers and all the ins-and-outs of what Serious difficulty has to offer.

To be brief and summarise:

• More enemies
  
• Tougher enemies spawn
  
• Double ammunition
  
• 100% more damage output from enemies
  
• Item traps are even more dangerous
  
• 25% faster enemy reaction time
  
• 25% faster enemy movement
  
• Self damage is deadly

REVOLVER(S)[www.serioussam.wikia.com]
Damage: 10
Ammo: Infinite (reloading every 6-12 shots)
Effectiveness: Low
With the knife, this is the first weapon you start off with. Speaking about the revolver 'alone' is redundant, as its partner is literally 10 seconds to the right of the first level in The First Encounter, and 10 seconds behind you across the lake in The Second Encounter.


When pairing the revolvers together, they do make for a formidable weapon, albeit useless beyond the first few levels other than the few 'rare' circumstances you may come across.
At times, enemies might just be so far away that you can kill them with the revolvers before they're near you, and due to the revolvers' accuracy, you're wasting no time, or ammo, killing them.

This weapon is the most versatile long-range weapon until you get a hold of the Tommy Gun, which works wonders on weak enemies such as:

• Beheaded enemies
  
• Kamikazes
  
• Gnaars
  
• Kleers (until they get close)
  
• Marsh-hoppers (until they get close)
  
• Minor (Blue) Biomechanoids
  
• Harpies (until they get close)
  
• Electro-fish

Now that may sound like a well-rounded weapon considering the amounts of enemies it can take on, but this is all very circumstantial and in no way ideal against any horde remotely large as the DPS is too low. Think of this weapon as your sniper rifle for weak enemies.
This weapon is perfectly, irrefutably accurate, and it fires 6 shots before requiring a reload. A great tactic to remember is that if you switch to another weapon, your revolvers will have replenished their ammo, meaning that if you switch weapons after 5 shots then switch back, you're saving more time.

For the sake of practicality, sometimes I may recommend using only one revolver as opposed to two, because shooting twice with one click can be very distracting, and you can find yourself managing to miss long-range targets. This is a personal problem of mine, but others may share it.


TOMMY GUN[www.serioussam.wikia.com]
Damage: 10
Ammo: 999
Effectiveness: Medium
Finally, your first fully automatic weapon, presented to you in the form of what's quite possibly the coolest submachine gun ever created!
Doing the same damage and having the same accuracy as the revolver, it's seen that the Tommy Gun has an incredibly fast fire-rate in comparison, and essentially replaces the revolvers for everything they're worth.
The only con that lies within this weapon is that early on there's hardly any ammo for it to spare.
I recommend that you keep it on reserve for a while, using it only to take down distant targets, or small hordes of weak enemies.

This weapon is key in defeating hordes of enemies that are deadly at contact, and die in a few shots. So by that, I mean specifically Kamikaze, Electro-Fish and Marsh-Hoppers. The stability of the Tommy Gun, mixed with its ability to fire the instant you click, makes the weapon incredibly versatile against hordes of said enemies.
Other than that, the Tommy Gun is great at causing enemies to flinch. Much like DOOM's pain-meter, Serious Sam also happens to have a sort of 'flinch'-meter, where if you pass over a certain damage threshold, the enemy will stop and give off an agonising sound with an animation.
I'm unsure if this works consistently on larger enemies (such as Biomechanoids/Reptiloids), but it works wonders on Kamikaze, Beheaded, Kamikaze, Electro-Fish, Kleers, Harpies and Arachnoids.
Thanks to flinching, you can build your own confidence when taking down hordes of enemies. It allows you to prioritise, and it allows you to even stand still and know it won't hit you before it dies.
As I said earlier, flinching will be discussed later.


MINIGUN[www.serioussam.wikia.com]
Damage: 20
Ammo: 999
Effectiveness: High
This weapon proves itself the moment you get it, dishing out twice the amount of damage of any bullet-based weapon (revolver/tommy gun), and giving overall one of the highest amounts of DPS per individual target. It chews through ammo like it's eating a really damn good chocolate bar, but its capabilities are endless if you're stocked up on enough ammo.
The minigun is recommended for any situation with a mixture of weak-to-strong enemies are attacking, allowing you to effectively prioritise enemies and keep them at a distance. Although bullet ammo is incredibly valuable, it's not particularly 'sparse', meaning as long as you don't overuse it, you should be fine.

This weapon's use shines highly on defending yourself from large enemy projectiles, such as shooting the rockets from Biomechanoids and the heat-seeking balls of Reptiloids, while maintaining focus on the enemy itself.


SNIPER RIFLE[www.serioussam.wikia.com] (TSE only)
Damage: 70 (unzoomed)/300 (zoomed)
Ammo: 100
Effectiveness: High
This is your one and only, truly long-range weapon -- ignoring that all other bullet-based weapons hit the target instantly from any distance.
Very little is to be said about this weapon, other than that you must always zoom in when targetting larger enemies or distant enemies, otherwise you're essentially wasting valuable Rifle ammunition. Know that if you're caught with your pants down and this weapon out, it's still viable to use on the few targets surrounding you if they have low health, but it's still not recommended as the fire-rate is incredibly slow and will lead to absolute danger if you miss.
Although the '300 damage' looks very luscious, know that a Rocket Launcher may still be more viable for mid-to-long range targets as the fire rate is definitely a lot faster.

Paired with Serious Damage, this weapon is recommended to be used against any high-threat enemies like a Reptiloid Demon, Red Biomechanoid, Green Reptiloid or Red Arachnoid as its damage is impeccable.
With this, it's also very useful to be used against Harpies due to simplicity and ammo conservation instead of wasting bullet ammo from the Minigun.

[/previewimg]PUMP-ACTION SHOTGUN[www.serioussam.wikia.com]
Damage: 70 close-range/~10-30 medium-range
Ammo: 200
Effectiveness: Low-Mid
Providing 7 pellets at 10 damage each, this is the first of two shotguns in Serious Sam.
Although this weapon is later doomed to rest on Sam's "screw it, I have bigger guns now" shelf, it proves itself to be incredibly useful early on in the game, and just about any other time you're in-doors. The pump action shotgun ensures that any low-health enemy, such as those that were listed under revolver, will die in 1-3 hits from short-medium range.
This weapon works best under light hordes (<15), or when poking your head around corners to take a shot at a Blue Biomech or perhaps a Young (Yellow) Arachnoid to get some safe damage in.

Thanks to the spread of this weapon, as well as its moderately high fire-rate, it's recommended to use against Kamikaze, Marsh-Hoppers and Electro-Fish hordes early in the game if you don't have a fully automatic weapon at your disposal. This is due to a nice spread of moderate damage, as well as the 'stun' factor which can cause the enemy to flinch (this will be discussed later).
Don't shy on the use of it, as shotgun ammo is the most common source of ammunition early game.


DOUBLE BARREL SHOTGUN[www.serioussam.wikia.com]
Damage: 140 close-range/~10-40 medium-range
Ammo: 200
Effectiveness: Medium
This is your strongest fist, clenched and ready to launch. What it lacks in fire-rate, it makes up for in a strong, well-placed hit.
Be sure to get used to this weapon early on in the game, as it's the most effective weapon against harpies, biomechanoids, arachnoids, werebulls and especially kleers.

Due to its different bullet-spread pattern, it's just as ineffective at medium-range as the Pump-Action Shotgun is. With this, you should know to never bother using the Double-Barrel Shotgun unless the target is either large, or charging at you.
This shotgun is able to defeat several weak enemies at a time, is able to take kleers and harpies down in one shot at point blank, and is able to two-shot any werebull if you dodge it correctly.

Timing is perhaps the most essential skill to acquire when using the Double Barrel Shotgun, as it requires enemies to be very close to get the most out of its damage, and then you're stuck with the long reloading time inbetween, making you completely vulnerable.
Once you've gotten the hang of this, it's a very formidable weapon, but know that neither of the shotguns are in any way practical or appropriate beyond the 'half-way' mark of the game, when hordes pick up in speed and size.

ROCKET LAUNCHER[www.serioussam.wikia.com]
Damage: 100 (+ 0-50 blast radius)
Ammo: 100
Effectiveness: High
On-par with the Minigun, the Rocket Launcher is the most commonly used weapon in the entire Serious Sam franchise, due to its high ammo spawn and high practicality during fights.
Predominantly effective is its use in small/medium fights against weak/medium strength enemies, where its splash damage and fire rate is put to effective use.
Although it's essentially incapable of holding back a horde of Kleers, it's capable of two-hitting Werebulls, wiping out clusters of Kamikazes, Marsh-Hoppers, Electro-Fish and even Harpies with some well-placed shots.
It's the alter-ego of the Minigun, both being great at everything, except the Rocket Launcher is arguably better at taking down large enemies, whereas the minigun is better at taking down hordes. It is advised to shoot an extra shot or two at Red Biomechanoids or Reptiloids, as sometimes their projectiles might make contact with your rockets, essentially deflecting the damage.

Another benefit of the Rocket Launcher is that it fires projectiles, not bullets. This allows you to jump from cover to fire a few shots without being hurt by any kind of Biomechanoids, Beheaded enemies or Arachnoids. You can also learn how many shots may kill a strong enemy, fire that many at said enemy, then move on and attack another enemy, knowing that the previous enemy will die once the rockets reach their target.
It is recommended the Rocket Launcher against Highlander Reptiloids due to the consistent damage, which most other weapons don't have for such a fight.

And another thing: please be wary of explosive damage.


GRENADE LAUNCHER[www.serioussam.wikia.com]
Damage:75 (+ 0-100 blast radius) (+ 1-4 charge)
Ammo: 100
Effectiveness: High
By far the most underrated weapon in Serious Sam as a whole, usually going entirely unused because the Rocket Launcher "works well enough".
Honestly, fair call. The Rocket Launcher does have more perks, such as higher direct damage and without the need to charge the shot, but I want to explain the benefits of the Grenade Launcher.
The Grenade Launcher uses its own specific ammo, meaning it isn't handicapped by the use of another weapon like the Minigun/Tommy Gun or Shotgun combo, allowing it to be used at any time without an issue.
With this, it also does more damage (175-179 compared to 150) than the Rocket Launcher, meaning it's more confident in damaging enemies, and can one-hit the Cucurbito.

In comparison to the Rocket Launcher, the Grenade Launcher has an "as fast as you can click" fire rate, creating almost literal carpet bombs, and it also has an insanely fast projectile speed if fully charged, making it incredibly useful against distant Biomechanoids, Werebulls, Kleers, Arachnoids or Reptiloids as well as hordes of enemies. Learn how to use this weapon, and learn how to love it, because most ignore it wholeheartedly.


LASERGUN[www.serioussam.wikia.com]
Damage: 20
Ammo: 800
Effectiveness: High
This weapon is a little odd to discuss, as its rate of fire is half of the Minigun's, it's projectile-based and a lot slower than the Minigun, and it only does the same amount of damage as the Minigun.
It'd be clear to say it's less effective than the Minigun, and in most circumstances it is.
But it has its own perks, perks such as the bullet drag allowing you to spread out damage easier when you strafe, and its ability to shoot down gigantic projectiles such as ones which the Highlander Reptiloid throws, or the ones which Lava Golems hurl with ease, making it a formidable weapon against larger, deadlier enemies that you will encounter.
These are the only two reasons I could state that make it effectively better than the Minigun, but in most cases, you should stick to the Minigun.
I only recommend it when you wish to save Minigun ammo, or when fighting Demons/Highlanders/Lava Golems.


CANNON[www.serioussam.wikia.com]
Damage:500 (750 fully charged)
Ammo: 60
Effectiveness: Very High
The BFG of Serious Sam. The key weapon in destroying anything and everything.
The Cannon provides a strong fire rate, insane amounts of damage, a fast projectile speed and can wipe just about anything.
This weapon needs very little explaining, but it requires a lot of practice to use it to its full potential.

Things to make note of are that you do not always need to fully charge the weapon for it to do a lot of damage, as even without charge it can pierce through several enemies effortlessly. Charging is recommended for wiping massive waves of enemies, or to one-hit any large enemy such as Red Biomechanoids, Medium (not Big) Lava Golems, Demon/Green Reptiloids as well as Adult (Red) and Juvenile (Yellow) Arachnoids.
It is also recommended to fully charge your cannonballs and always use this weapon against Big Lava Golems, Highlander Reptiloids and the Final Boss, as the DPS is by far the highest and most effective in the game, and its ammo should be used wisely.

MILITARY KNIFE[www.serioussam.wikia.com]
Damage: 120
Ammo: Infinite
Effectiveness: Low
Although the Military Knife carries on more DPS than even the Double Barrel Shotgun, it is not recommended to be used under any particular circumstance in Serious difficulty.
When paired with Serious Damage, the output is enough to take down tough enemies, but it's not justifiable in Serious difficulty due to most enemies having a devastating melee attack.

A note to point out about the knife is how exactly Sam uses it. In a single animation, he is seen swinging it in one direction, and then the other. As far as I have seen, this actually acts as two hits instead of one, despite it being a single animation.
In short, this means that if you can keep the full animation on an enemy, you're doing 240 damage instead of 120.


CHAINSAW[www.serioussam.wikia.com] (TSE only)
Damage: 25 (per second)
Ammo: Infinite
Effectiveness: Low
This big, bad 'knife replacement' will let you carve figure-eights into any weak enemy you come across, with enough range and 'flinch' capabilities to leave you out of harms way when taking on small amounts of enemies.
In some circumstances, yes, it is practical, but in most it is not.
It can take down any Beheaded enemy or Gnaar in moments, as well as Kleers and (if you're daring) Harpies, but it's simply not worth the effort or risk when taking on several enemies.

Believe it or not, this weapon is insanely effective at destroying hordes of Marsh-Hoppers, as the range of the chainsaw, mixed with your ability to circle-strafe and kite, renders them all turning into a mushy, green puddle of nothing. Note this is high-risk, but it conserves a lot of ammo.


FLAMETHROWER[www.serioussam.wikia.com] (TSE only)
Damage: 10 per flame (+ burn damage)
Ammo: 999
Effectiveness:High/Very-High
By far one of the most practical and essential weapons gifted to you in Serious Sam, and should it be used in the right circumstance, very little will ever get in your way.
It comes with a few cons, but they're outweighed heavily by the large amount of benefits it brings.
For any low-threat enemy, they'll be incinerated immediately, even a single click is enough to have them burn to death.
This is the best weapon at causing enemies to flinch, as the burn damage renders most enemies useless. A single click on a kamikaze can have them stop in their tracks and explode due to the burn damage, but giving you enough time to back off.
It works very well against Kleers, allowing you to damage several at a time without having to prioritise and target a single one, and this goes for Harpies, Werebulls and Marsh-Hoppers.

Don't over-use this weapon, but don't under-estimate its abilities. I recommend to use it mostly in-doors due to its lack of distance, and I recommend to use it on close, strong enemies when paired with Serious Damage.

Please note that it does not harm Lava Golems of any sort.


SERIOUS BOMB[www.serioussam.wikia.com] (TSE only)
Damage: Instantenous death of all non-boss enemies
Ammo: 3
Effectiveness: Obliteration of all enemies within 700 feet/215 meters.
This is your second-life, and there is very little to say about it other than it'll save you in a dire situation.
I don't recommend to use this at any time unless you're stuck dying at a poor save point, and don't have enough health/armour/ammo to counter the amount of enemies. Use wisely, as they're incredibly rare to come across.
Be sure to conserve as many as you can, as you (if you're good enough) can use several at a time just to get a load off during a ridiculous fight, of which there a few in The Second Encounter.

BEHEADED ENEMIESROCKETEER[www.serioussam.wikia.com]
Health: 19
Damage: 10 ranged/20 melee
Points: 200
Tactics: Side strafe their simple attack, using any easy-fire weapon to take them down in one/two shots. Any projectile-based weapon is overdoing the damage and wasting ammo.


FIRECRACKER[www.serioussam.wikia.com]
Health: 19
Damage: 8 ranged/40 melee
Points: 200
Tactics: Like the Rocketeer, except several shots are spread across the ground. Dodge by stepping inbetween the shots, or by simply jumping over them. Use a weapon generous with ammo.

BOMBER[www.serioussam.wikia.com]
Health: 19
Damage: 20 direct impact (+ 0-30 radius)
Points: 500
Tactics: Like the rest, except throws a single bomb every now and again. Avoid these, and the explosion radius, by jumping away from the where the bomb is landing. Never stand in the same spot, and use a weapon that has a lot of ammo.


ZORG
MERCENARY[www.serioussam.wikia.com] (TSE only)
Health: 40
Damage: 20 ranged
Points: 500
Tactics: Never a priority target in a big fight as their shots can be jumped over and strafed easily; use whatever weapon is at your disposal to remove them during the fight with minimal focus, like picking them off with the Minigun while you go to aim for a bigger enemy.



COMMANDER[www.serioussam.wikia.com] (TSE only)
Health: 60
Damage: 20 ranged
Points: 800
Tactics: Very similar to the Mercenary counterpart, except rarer (usually one per pack), slightly more health, and more projectiles fired in a single shot. Dispose of them the same way you should with the Mercenaries: quickly and effectively.


GNAAR
MALE GNAAR[www.serioussam.wikia.com]
Health: 60
Damage: 7
Points: 200
Tactics: Can be taken down in a few shots by a Tommy Gun, or by a single Pump-Action Shotgun shot. Their attacks can be manipulated by jumping over them and sometimes balancing on their head to avoid damage. Easy to avoid.


Note: Male Gnaars can come in transparent and floating forms. These have the exact same statistics as the basic Male Gnaar, but are just a little harder to defeat. Be wary of using explosives if you notice one.


FEMALE GNAAR[www.serioussam.wikia.com]
Health: 90
Damage: 7
Points: 500
Tactics: Can't be taken down in a single Pump-Action Shotgun shot, but will definitely die to a single Double Barrel shot. Exact same tactics may be used on the female from the male, meaning you can still jump on their heads to dodge their attacks.

Note: Female Gnaars can come in transparent and floating forms. These have the exact same statistics as the basic Female Gnaar, but are just a little harder to defeat. Be wary of using explosives if you notice one.


WITCH-HARPY[www.serioussam.wikia.com]
Health: 100
Damage: 16 ranged/40 melee
Points: 1,000
Tactics: Deadly alone? No. Deadly in packs? Hardly. Absolutely dastardly when surrounding you or existing during horde fights consisting of melee/charging focused enemies? Yeah.

These are the only enemies in Serious Sam which are airborne, ignoring Red Biomechanoid rocket projectiles and Reptiloid projectiles being a little over head. Harpies will predominantly be seen flying in groups, launching bright projectiles usually at the same time (due to spawn).
Harpies generally spawn very far away in horde fights, meaning they won't be an issue if you keep the horde settled on ground. The problem may arise when the Harpies get close, as their melee is very difficult to avoid, even by strafing.

When facing Harpies as a lone pack of enemies, use any weapon you desire as Revolvers can do a decent amount of damage when they're far, and a Double Barrel Shotgun is enough to one-hit them before they swoop. Note that if you're using the Double Barrel, be sure to back-peddle if they're going in to swoop so that you may have more space and time to reload.

When facing Harpies in addition to a large horde, try to ignore them if you're in an uncomfortable space, and then open fire on them with preferably the Lasergun or Minigun, as there's no time to be careful with ammo, and you should generally be well-stocked in these particular fights.

ELECTRO-FISH[www.serioussam.wikia.com]
Health: 30
Damage: 30 melee/30 ranged
Points: 500
Tactics: A very intimidating, yet luckily rare, enemy that will always surprise you, even if they only ever spawn in the water. They have a surprisingly long attack range, spawn in packs, move incredibly fast, and will hit you even if you're on land, sometimes even actually swimming up onto land to get that little bit closer.
With such low base health, I recommend using any automatic weapon (preferably the Tommy Gun) to take down packs of Electro-Fish. Some say that using explosives such as the Rocket Launcher or the Grenade Launcher (I'd say Grenade Launcher due to higher explosive radius damage), can wipe out packs of them entirely, but I believe this can become very inconsistent as it's possible to miss, and due to their speed and getting up in your face, you're prone to damaging yourself.

Whenever you're in water, keep your ears ready for their gurgled spawning sound, as it's very difficult to notice them until they're too close for comfort.

MARSH HOPPER[www.serioussam.wikia.com]
Health: 9.5
Damage: 20 direct impact
Points: 500
Tactics: An enemy so weak, yet so dense in quantity, that one could compare them to a green wave of imminent death if not dealt with immediately. Marsh Hoppers will always spawn usually surprisingly, quickly and aggressively. Sometimes they will appear in relatively small packs in-doors to keep you on your toes, and in gigantic, boss-health-massing packs.

In most environments, the Tommy Gun will make short work of them, and a Rocket Launcher is capable of taking out globs of them effortlessly (but you run the risk of damaging yourself) in any situation. But when you encounter the absolutely massive packs, it's recommended to use the Minigun to mow down any oncoming Marsh Hoppers, and I find it hard to recommend any alternative due to how efficient the Minigun is.

KAMIKAZE[www.serioussam.wikia.com]
Health: 19
Damage: 0-60 blast radius/60 direct contact
Points: 2,500
Tactics: One of the key signature enemies of the entire Serious Sam franchise, its infamous title being obtained through their brutality, density and Sam's grumpy retort upon first defeating one of the explosive, screaming goons.

On Serious difficulty, you're not going to live if more than a few ram into you, no matter what your health and armour may be. You cannot outrun them, so your best defense is a good offense, and it's recommended to use essentially anything in your arsenal.
Due to their low health, high numbers, and large explosive radius, it's appropriate to use almost anything from Shotgun, to Tommy Gun, to Minigun, to Lasergun, to Rocket Launcher, to Grenade Launcher. They are very easy to handle when alone, but will be one of your most fearsome challenges when close-by, coming from all directions, or hidden between hordes of different enemies.

Early game, when you only encounter a few at a time, or in any situation similar, I recommend that you use one blast of a Flamethrower, or one shot of a Revolver if they're too close, as this will cause them to intermittently flinch and give you enough space between them to open fire and take no explosive radius damage.
Other than this, you only simply keep your distance, manage your gunfire, and be careful.

"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA-"

KLEER[www.serioussam.wikia.com]
Health: 130
Damage: 20 ranged/40 melee
Points: 1,000
Tactics: Possibly the most difficult enemy for the new player to conquer due to its vast mobility and capabilities to harm the player.
Kleers are fast, have a neat chunk of health, painful, surprising, and come in packs ranging from small to very, very large.

Early game it is recommended to use the Double Barrel Shotgun at absolute point-blank range (which does 140 damage, winning out the Kleer by 10 damage). This is easier said than done though, as the only time the Kleer is close enough is when you're quite literally less than a meter from it. This means you either have to run into a Kleer after it has lept to kill it, or shoot it while it's lunging at you.
The latter is what I recommend, as it's much more efficient, but it's a lot harder. You will have to recognise their sound cue and time it to your shot. When you do this, still work to strafe the leap in case you fire too early/late and the Kleer lives.
Later on, it's recommended to use either the Grenade Launcher, Minigun or Flamethrower for taking out packs of Kleers due to their damage output.

One other thing you must learn is how to continuously dodge Kleers when focusing larger targets such as a Red Biomechanoid for example. When a Kleer leaps at you and you dodge it, you're given roughly 1.5 seconds before the Kleer recovers and turns around to attack you again. To master the dodging, I recommend to keep the Kleer(s) in your peripheral vision when playing in Third Person, or to count in your head when they're about to turn back (often noticed by the sound of their clopping steps). When a Kleer is coming back to jump at you again from behind, try to remember which direction it lept to, and try to counter that by strafing in the opposite direction.

This strategy is key for most advanced players, as it can be done (if done well enough) to dodge almost any and all Kleers that are after you when you're busy focusing a larger enemy.

CUCURBITO[www.serioussam.wikia.com] (TSE only)
Health: 175
Damage: 40 melee
Points: 1,500
Tactics: Think of these guys as deadlier Kleers, almost in every way.
Their large chunk of health makes them impossible to one-hit with a Double Barrel Shotgun or Rocket Launcher, which can be of concern as most enemies it runs around with can be, making situations difficult.
Unlike Kleers the Cucurbito will not 'leap', they will only charge to your location, and if they connect they will proceed to excessively cut at you rapidly. This can mean a very quick death, but luckily they're relatively easy to dodge by sidestepping unless in enclosed areas.
Treat their charge attack like you would a charging Werebull, and be ready to have them run back once they've run past.

My preferred way to deal with the Cucurbito is to fire a single shot of the Grenade Launcher, which does 175-179 total damage -- one-hitting them. Most people tend to ignore this as Rocket Launcher is generally people's first choice, despite lower damage. Due to their large size, they're prone to be taken down by fully automatic weapons such as the Lasergun, Flamethrower, Tommy Gun or Minigun, which can take down most packs of Cucurbito without any issues.

WEREBULL[www.serioussam.wikia.com]
Health: 250
Damage: 40 charge/40 gore
Points: 2,000
Tactics: Werebulls are made to wreak havoc, as they're always accompanied by a variety of enemies, mixing up the pacing; making it difficult to know which enemy to be targetting due to the differences in speed and health.
They are one of the fastest enemies in the game, and do more damage depending on how direct their charge was. Although they're quite simple to dodge, they can generally limit your options when it comes to maneuvering the battlefield.

Early in the game, it's recommended to use the Double Barrel Shotgun, as two direct shots will take a Werebull down. As long as you have the distance, two rockets will also do the trick.
For much larger fights, the versatility of the Minigun will do well enough in keeping them back into the density of the horde.
It's also recommended to use the Flamethrower if you're desperate, as they're a charging enemy, meaning you're not going to miss. A lone shot from the Sniper Rifle is enough to drop a Werebull from long-range, which is great for picking them off before the fight comes to you.
There's very little difficulty about the werebull, you just need to always make sure they're not dominating the fight by putting you into a (figurative) corner.

ZUMB'UL[www.serioussam.wikia.com] (TSE only)
Health: 210
Damage: 20 (direct impact) + 0-20 (blast radius)/20 melee
Points: 3,000
Tactics: Deadly, duel-firing rocketeers that are generally placed at a distance either far away, or in the back of enemy hordes.
Although they're easy to dodge when you're aware of them, their primary strength comes from their large projectile size and speed, making it difficult to dodge when you're not aware of their location.
The best weapon to use when targetting a particular Zumb'ul is definitely the Sniper Rifle, as it will immediately remove them from a fight at medium-to-long range, making their projectiles not a hassle. If for whatever reason you cannot use a Sniper Rifle, the Rocket Launcher or Minigun are viable options, as you don't have to prioritise them entirely, and you will be able to shoot down any projectiles that they have fired at you.

As for the projectiles themselves, know that one of the two fired will always be heading directly towards you, while the other will be off to the side to catch you if you're running in that direction.
If you're unable to dodge the rockets by strafing, know that it's possible to jump over the rockets if timed correctly.

LAVA GOLEM[www.serioussam.wikia.com]
Health: 10,000 Large/800 Medium/100 Small
Damage: 40 melee (L/M) + 0-20 (Shockwave)
20 melee (S) + 0-10 (Shockwave)
40 ranged (L)/30 ranged (M)/14 ranged (S)

Points: 50,000 (L)/2,500 (M)/500 (S)
Tactics: The most 'complex' enemy type in Serious Sam, capable of splitting apart into smaller forms ranging three sizes. Each time it's destroyed, it will split into several smaller forms.
Although the large one has a lot of health, they're not all that more dangerous to their medium counterparts, which do an identical amount of damage.
It's simple to kill any small lava golems using a single shot from a rocket, grenade or 10 shots from a Tommy Gun -- simple.
The medium are possibly the most threatening, as usually they're paired in twos or threes, doing huge amounts of damage if they get a hit on you. They will take a handful of explosives or shots from the Minigun to take down, but are not threatening enough to prioritise over other enemies such as Kamikazes, Kleers or, Cucurbito or Werebulls if they're close.
Large Lava Golems are very rare to come across, appearing primarily as their own boss. Know that their attacks are not as deadly as you may think, but due to their great size it's recommended to stay far, far back, using charged cannonballs or rockets to destroy them.

All Lava Golems throw lava rocks, and they will predict your position before they throw. To counter this, watch for the moment they throw the rock, then break the direction you're running in and go another way. If for whatever reason you cannot run away from the rocks, know that they can be destroyed easily.

Note: All Lava Golems are immune to the Flamethrower (for obvious reasons).

BIOMECHANOIDS
MINOR BIOMECHANOID[www.serioussam.wikia.com]
Health: 150
Damage: 10 ranged
Points: 2,000
Tactics: On Serious difficulty, these enemies are often gone underestimated with the damage they're capable of. With higher difficulties, the firerate of the Minor (Blue) Biomechanoid increases, making its laser volleys more frequent. If these enemies go unchecked and ignored, the insanely fast projectile speed will catch up.
To avoid this, it's possible to jump over the lasers, and they will never hit you as long as you're strafing in one direction.
They're impossible to one-hit with a Double Barrel Shotgun, but one shot with Grenade Launcher or Rocket Launcher, or even some shots from the Tommy Gun/Minigun, and they'll be down instantly.

MAJOR BIOMECHANOID[www.serioussam.wikia.com]
Health: 750
Damage: 100 ranged + 0-50 (blast radius)
Points: 7,500
Tactics: Possibly the most formidable high-end enemy that's not regarded as a boss, up there with Adult Arachnoids and Medium Lava Golems. If you're knowledgable enough, these enemies can be easier to handle than you could imagine.

Major (Red) Biomechanoids fire twice subsequently; every time they fire it's from their left launcher (their perspective) then their right launcher. Both of these rockets are able to be destroyed.
To combat this in a tight situation where dodging is too difficult, it's possible to use a Tommy Gun/Laser Gun/Minigun and shoot at their left launcher until you see an explosion, then immediately target their right launcher. This will render them harmless and being unable to fire any projectiles, while taking damage the whole time.
Predominantly, this is the only tactic you should have to know, anything else can be solved by strafing.

ARACHNOIDS
JUVENILE ARACHNOIDS[www.serioussam.wikia.com]
Health: 300
Damage: 20 ranged/40 melee
Points: 1,000
Tactics: A relatively commonplace enemy indoors and outdoors in large fights; their primary goal is to make a battle situation uncomfortable, and to whittle down your health if you don't pay immediate attention to them. Their ranged attack has the fastest projectile speed in the entire game, almost working like a hitscan when they're aiming at you.
When facing a Juvenile Arachnoid indoors, it's recommended to use shots from your Rocket Launcher or Grenade Launcher when diving in and out of cover, but you can also use a Tommy Gun or Minigun to cause them to flinch if you maintain focus fire, rendering them harmless.

ADULT ARACHNOIDS[www.serioussam.wikia.com]
Health: 600
Damage: 20 ranged/80 melee
Points: 5,000
Tactics: Although they're much larger, redder, meaner and stronger, Adult Arachnoids aren't that much more of a threat. Their bullet damage remains identical, but their health and melee damage is doubled. On topic of the melee damage, it's something that can be left undiscussed, as there's never a reason to get close to an Arachnoid and should always be avoided.
The exact same tactics should be applied to an Adult Arachnoid as a Juvenile Arachnoid, as the Minigun is enough to cause flinching, and shooting rockets/grenades from cover will still keep you safe from their insanely fast bullet speed.

COMMON REPTILOID[www.serioussam.wikia.com]
Health: 400
Damage: 20 projectile/80 melee
Points: 5,000
Tactics: A commonplace enemy that conjures a homing, destructable projectile at the player. These projectiles will last 10~ seconds, usually enough time to circle back and attempt to hit the player 2-3 times before exploding on their own.
There is a strategy that many advanced players used, and it's strategic use of strafing. When strafing right (for example), if the projectile goes past the left of you, then it's going to predominantly turn back and come at you to the right (going clockwise to counter-clockwise). So to counter this, sharply strafe right so that the ball won't be able to curve hard enough, or run backwards and too the left so the ball can't curve enough.
Now, when this projectile is flying about, you don't want to be looking at it due to focusing on other enemies, but you also don't want to be hit by it, so this is why the strategy is used.
The projectiles can be destroyed quite easily, but this means you will lose your focus fire on the enemy itself, which can create difficulty.
When fighting Common Reptiloids, Rockets and the Minigun are recommended, as both can destroy oncoming projectiles as well as provide consistent damage. If you're using the Rocket Launcher, be sure to shoot one or two more times than you need to incase the projectiles block the rocket.

HIGHLANDER REPTILOID[www.serioussam.wikia.com]
Health: 2000(TFE)/3000(TSE)
Damage: 40 projectile/160 melee
Points: 25,000
Tactics: This is renown as the 'boss' of the game if you ignore the final boss itself. It's a huge enemy, and is required to be defeated every time you encounter it.
Although it may look intimidating, it hardly ever comes with enemies (unless in co-op), and its gigantic projectiles are easy to dodge if you continue to jump or strafe in a particular direction.
Its projectiles can be destroyed, but it's not recommended as they will note home hard enough, and will explode on imact when hitting a wall.
It's recommended to use a Rocket Launcher, as Highlander Reptiloids are generally far away, meaning charging your Grenade Launcher will take too long, and will waste too much of your minigun ammo otherwise. If you have a cannon, be sure to always use it on Highlander Reptiloids, and always to charge the cannonball.

DEMON REPTILOID[www.serioussam.wikia.com] (TSE only)
Health: 500
Damage: 40 projectile/100 melee
Points: 5,000
Tactics: By far my most hated enemy in the entire franchise, though many will disagree.
Insanely fast, Highlander-like projectiles which are capable of ridiculously good homing capabilities, being able to hit you from behind like Common Reptiloids. What's worse is that these projectiles are immune to bullets, and are only able to be destroyed by the Lasergun, Cannon, Rocket Launcher or Grenade Launcher.
I recommend to stay near close cover where you can be protected on at least two sides, so that the projectile may be blocked. Demon Reptiloids are a high-priority target and should predominantly removed from battle first, as they will be throwing projectiles the whole fight, and are almost always incredibly far away.
To destroy them, shoot a volley of rockets (as the enemy rarely ever moves), a well-placed charged cannonball, the minigun, or several shots from the Sniper Rifle to take one down.

Within Serious Sam's plain level design, there are a three distinguishable environments that you can apply to almost every tactical situation. Each particular environment will generall have a different set of enemies in a different placement, proving different challenges.

Early on in Serious Sam, and a little near the end stretch, a lot of time is spent indoors.
Indoors creates drastic claustrophobia, limiting your capabilities of strafing large groups of enemies, and leaving you to rely on your quick-snap aiming abilities when encountering traps and corners.
When indoors, make good use of alternating between both first and third person views. Sometimes, rooms will be soo small, or doorways will be so tight that being in third person will limit your view on what's in the next room. But in larger areas with corners, using third person will allow you to rotate the camera while still remaining around the corner, allowing you to see what exactly is around the corner.

When indoors, be wary of:

• Traps
  
• Enemies on high ledges
  
• Enemies around tight corners
  
• Secret holes/walls for weapons/health/armour
  
• Ways to backtrack when outnumbered

Where some of the biggest fight happens, arena fights generally escalate from weaker enemies to significantly stronger enemies. The hardest thing to deal with in an arena fight is that enemies will almost always come from several directions are once, causing huge amounts of confusion if you don't take control of the situation immediately.
Depending on the strength and density of the enemies, it's possible to hold your ground in the middle, but it's preferable to instead shoot your way into a portion of the arena, which will allow you to kill enemies which spawn closer, and give you more distance from the other enemies.

When in arenas, be wary of:

• Any enemies that may be high or far in the distance, and are projectile-based
  
• What enemy is faster and which will reach you quicker
  
• What to prioritise immediately
  
• What opportunities you have to dodge when strafing
  
• What enemies are behind you, and timing when to dodge them

Seen as the most difficult environment to handle in Serious Sam amongst most players are the long stretches of land in which enemies will spawn and essentially flood to push you back.
This is where Serious Sam is remembered for S+M1, because with so many enemies, you're left having to run backwards more than running forward.

Like a war of attrition, it's your job to wear down the horde by prioritising and aiming both quickly and efficiently. Targetting any high, distant, ranged targets should be your first priority.
These range from Biomechanoids (although they're usually ground-based), Arachnoids, Reptiloids and Harpies.
Follow it up by weighing out the gunfire between both fast, incoming units, and larger, slower units. If you start shooting at a Major Biomechanoid, this doesn't mean you have to kill it -- as long as you're harming it, you're doing enough.
If you keep this consistency, and keep running forward when the opportunity presents itself, you will be able to push back the enemies and succeed.

When in corridors, be wary of:

• Any enemies that are high up/in the distance
  
• Stronger, ground enemies
  
• Faster enemies
  
• Loose travelling projectiles
  
• What weapon to use at what distance for what enemies

Although there are no true, definite stategies imposed by Croteam themselves, there are several unique stategies that players have developed themselves, ranging from minor to significant. The ones I will list will be the ones I've developed myself, most of which come naturally to the player.

Whenever I play with friends, they always discuss how awkward switching to the appropriate weapon is. I also see fault in this, as you're having to press some keys (1-8) two times to access the weapon that appears below it on screen. So most of you reading this are either relying on scroll to get to your weapon, which can be very inaccurate and costs a lot of your focus, or are sometimes double-tapping a key to get the weapon you want.
Granted, Croteam have made the stronger, more appropriate weapons accessible through the first key press, such as the Minigun, Rocket Launcher, Double Barrel Shotgun, etc. but this can still cause a lot tension when needing to get another weapon out.
My solution? Map your keys around WASD (or ESDF). This is what plenty of players do in other classic shooters such as Quake, Unreal and even Doom.
If you're using WASD, key your most used weapons (usually Minigun and Rocket Launcher) to Q and E, and make your other strong, key weapons bound to ZXC and/or RFV.
Personally, I have Q for Rocket Launcher, E for Minigun, R for Cannon, F for Flamethrower, and 1/2/3 for the melee/pistol/shotguns.
Figure out what works best for you, and you'll be able to almost instinctually select what weapon you want without having to lose focus on the enemies.

So many new players ignore that Serious Sam has a feature to be played in Third Person, simply by pressing 'H'.
The Third Person view allows for an insane FOV, letting you see far more to the left and right, as well as a bit behind you. In a game where a primary strategy is to prioritise, aim and dodge, having extra vision is a gift worth having.
The Third Person view will allow you to know more directly which way a projectile may be heading, which way a Werebull or Kleer may be charging, or simply a wider view of platforms which you can navigate to get around.

The cons of Third Person are generally seen indoors, as the ceiling or rooms sometimes aren't big enough to give you a sufficient view, and may in fact show less view, or annoyingly switch you between First Person and Third Person repeatedly. Along with this, Third Person in water can be very irritating as your crosshair will slowly float around the screen, which compensates for the First Person view which also floats around.
Another con is that when aiming at a high target, such as Demon Reptiloids or Harpies, is that your view can be obscured by objects on the ground -- generally grass -- which will block your view.

Generally frowned upon in most communities when being taken advantage of, but I believe it can be justified on higher difficulties, especially when Croteam have applied a "Rocket Jump 100 Times" achievement in SSHD:TFE.
Rocket Jumping will do a lot of damage on Serious difficulty, but it will allow you to make ridiculous jumps, getting you to safe ledges where you cannot be harmed, as well some skipping.
You can immediately see why someone would call it 'cheating', but it's entirely up to you whether or not you want to do it. Usually if you're in an area that you can't get past, there's a way to apply a rocket jump to make it easier.

Sometimes enemies will come from such a wide angle, that it's impossible to focus on them all at the same time. You will have to learn to distinguish them only by a quick glance, and then be able to remember their movements and what they'll be shooting when they're range-based.
If you learn this, you will be able to almost subconsciously know where to strafe and dodge, without having to focus on exactly what you're dodging, just the 'bulk' of it.

Sometimes a projectile from a Demon or Common Reptiloid may slip past you, and so might a Kleer, Cucurbito, Harpy, Werebull or Marsh Hopper. Although it's only one, this can throw of your focus fire almost entirely, and it's important to keep control of the situation when this happens.
If, for example, a kleer gets past you, do not actually turn around to kill it when facing a much larger horde, because then you're just closing the distance, opening yourself up to a lot more potential damage. Instead, keep focusing on the horde and strafe in the opposite direction in which it jumped at you, and then try to dodge it again so it jumps infront of you, getting back in your line of sight when combatting the horde.

It may sound very confusing at first, but it's something you'll grasp and eventually understand wholeheartedly through your playthroughs.

• Karnak
  Near the end of Karnak, in the area before the final fight in the courtyard where you have to fight a large wave of Marsh Hoppers, Kamikaze and Kleers, there's a simple skip you can use to avoid the entire fight with a rocket jump.
  As soon as you enter the area, there will be two light stands (circular bowls) on the left and right of you. By jumping on top of one of these, balancing (as the game won't lets you stand on it normally and will make you slide), then timing a rocket jump, you'll be able to land on the mantle of the doorframe, and run alongside the area on the walls, letting you jump into the final courtyard.
  
  
• Courtyards of Gilgamesh
  In the Courtyards of Gilgamesh level in The Second Encounter, as soon as you go outside you'll notice a platform in the center, surrounded by a spike pit. If you have enough health (which you can by obtaining the first secret), you can rocket jump on and off of this platform, collecting the idol and moving on, saving an insane amount of time.
  
  
• The Grand Cathedral
  In The Grand Cathedral, after the final horde fight (before the boss), be sure to wait for the Speed Boots and Serious Damage to respawn before heading over to the boss.
  Why? Because if you collect those items (in that order), and quickly race to the boss, you will have a few moments time to spam all your cannonballs onto the final boss with Serious Damage applied, making the fight a hell of a lot shorter!
  
  I may end up adding more to this if I remember any other time-saving methods to make fights/situations easier.
  

Serious Sam's co-op provides insane amounts of fun, but it also adds a lot more to the table, including both more enemies and stronger enemies. This can create behavioural patterns in enemies, and can create different scenarios. It's important to know -- even if you just want to have fun with a friend -- how to handle a situation when the odds are even worse.

Enemies in co-op generally work on a "first come, first serve" policy, where the first player in their line of sight will be the one who's attacked. This is just a short note to understand, so it's recommended to let whoever has the most health to go first, because clearly they'll be at risk, but they'll also have a higher chance to survive.

I've also noticed that the attack patterns in both Kleers and Cucurbito are deadlier given the circumstance. If a Kleer or Cucurbito happen to kill a player on impact, they will immediately attack the next player with no cooldown between their charge/leap. This can wipe out several players at a time if they're not careful.

It's not ridiculously different, but the amount of enemies in co-op can be deemed 'too much' for 1-2 players unless they're incredibly skilled. Generally things will seem the same as singleplayer, but there will always be a few additional enemies mixed in where you usually would not see them when indoors, sometimes you'll recognise a horde fight is going for a few minutes longer because the spawning has been extended, sometimes you'll notice a couple of Red Biomechanoids in a horde which they weren't in beforehand.
Most noticeably will be more occurences involving Highlander Reptiloids, and previous Highlander Reptiloids being paired with a twin, or with a wave of enemies.

A short note, but focus fire should be different for every individual player.
If a friend is firing at a Red Biomechanoid and so are you, yet there's a pack of Kleers after the both of you, go for the Kleers instead.
That example applies to any scenario with any amount of enemies. You should always be attacking something different than your friend is, unless there's only a few targets around. Watch each other's backs and keep everything at a safe distance.

As most players do in Serious Sam, we all jump around excitedly to hear that funny "hunf!" grunt, and some may have discovered that you can actually jump on top of a friend's head without slipping off. This can lead to one of you getting over an edge without needing to rocket jump, or perhaps working as a booster to get even higher on a rocket jump.
You and your friends also don't have to complete identical objectives, such as pressing buttons at the same time, or placing down objects at different locations.

This can cut time in half, and sometimes even allow one person to move on entirely while another stays back to activate the objective.

Serious Sam is an incredibly difficult game when playing on Serious, and I would personally say it's impossible for me to get through the game without using any secrets to my advantage, especially the ones which allow you to get weapons much, much earlier.

By referring here, you can get an entire list of every secret in Serious Sam: The First Encounter.
By referring here, you can get an entire list of every secret in Serious Sam: The Second Encounter.

By referring to Mauritsio's list, I will attempt to point out the significant, easy-to-obtain secrets that I believe every player on the Serious run should be acquiring to keep health, items and ammo up to scratch.

For The First Encounter:

• Hatshepsut - Secret #3 (!!!)
  
• Hatshepsut - Secret #7 (!!!)
  
• Sand Canyon - Secret #3
  
• Sand Canyon - Secret #4
  
• Tomb of Ramses III - Secret #2
  
• Tomb of Ramses III - Secret #8
  
• Valley of the Kings - Secret #2
  
• Oasis - Secret #1
  
• Oasis - Secret #3, #4
  
• Dunes - Secret #3
  
• Dunes - Secret #7
  
• Sewers - Secret #4
  
• Sewers - Secret #5
  
• Metropolis - Secret #1 (!!!)
  
• Karnak - Secret #3
  
• Karnak - Secret #4
  
• Karnak - Secret #5
  
• Karnak - Secret #8
  
• Luxor - Secret #6
  
• Luxor - Secret #7

For The Second Encounter:

• Sierra de Chiapas - Secret #1
  
• Sierra de Chiapas - Secret #4, #5, #6
  
• Sierra de Chiapas - Secret #11
  
• Valley of the Jaguar - Secret #1
  
• Valley of the Jaguar - Secret #2
  
• Valley of the Jaguar - Secret #4
  
• Valley of the Jaguar - Secret #7 (!!!)
  
• Valley of the Jaguar - Secret #8
  
• The City of the Gods - Secret #3, #4 (!!!)
  
• The City of the Gods - Secret #12
  
• The Pit - Secret #1
  
• The Pit - Secret #5
  
• The Pit - Secret #6
  
• The Pit - Secret #10, #11 (Risky!)
  
• Ziggurat - Secret #1
  
• Ziggurat - Secret #2 (Risky!)
  
• Ziggurat - Secret #7 (!!!)
  
• Ziggurat - Secret #8, #9
  
• Ziggurat - Secret #11
  
• The Elephant Atrium - Secret #1
  
• The Elephant Atrium - Secret #2, 3 (for fun and ammo)
  
• The Elephant Atrium - Secret #8
  
• Courtyards of Gilgamesh - Secret #1
  
• Courtyards of Gilgamesh - Secret #11
  
• Tower of Babel - Secret #2
  
• Tower of Babel - Secret #6
  
• Tower of Babel - Secret #7
  
• Tower of Babel - Secret #8
  
• Tower of Babel - Secret #10 (Risky!)
  
• Tower of Babel - Secret #11, #12
  
• The Citadel - Secret #1
  
• The Citadel - Secret #10
  
• The Citadel - Secret #11
  
• The Citadel - Secret #12 (!!!)
  
• The Citadel - Secret #15
  
• Land of the Damned - Secret #1
  
• Land of the Damned - Secret #2
  
• Land of the Damned - Secret #3
  
• Land of the Damned - Secret #13
  
• The Grand Cathedral - Secret #1
  
• The Grand Cathedral - Secret #2
  
• The Grand Cathedral - Secret #3 (Risky!)
  
• The Grand Cathedral - Secret #5 (Grab items on far left and far right first; grab middle last!)
  
• The Grand Cathedral - Secret #6
  
• The Grand Cathedral - Secret #7

The reason TSE has a significantly larger amount of secrets is because of a few reasons. One being that after every time you complete a set of levels, all your weapons are reverted to nothing and must be collected again. Another reason being that TSE has a lot more secrets, which are predominantly easy to obtain.
The "(!!!)" indicate weapons, or huge caches of items you will definitely want for a fight.

http://www.serioussam.wikia.com/wiki/Serious_Sam_Wiki
- The unofficial, primary Serious Sam wiki

Serious Sam HD: The First Encounter
Serious Sam HD: The Second Encounter

Personal experience
Second-hand community information