All of these tower builds boil down to:

1) Slow-locking the enemies in one very very cold hallway.
2) Repeatedly pushing them back with skills to keep them there.
3) AOE towers are a beautiful thing.

Important tricks:

• The first waves are the hardest. Lots of last-ditch Force Pushing and Laser Mining one or two heavy mobs. Once you get a Level 2 Kairos down, the worst is over.
  
• Slow Fields slow air units too. Pay attention where air units fly: one Slow Field can hit most if not all of them!

Honestly, that's probably all you need to read. I'll include a text guide anyway, but if I were you I'd just skip to the screenshots.

I typically used 2 slowing towers and 2 mid-to-long-range damage towers.


Obvious choices: Kairos and Slow Field. This is what creates your icy hallway.

ALWAYS bring Kairos.
ALWAYS bring Slow Field...EXCEPT on the rare map (eg, Gallery) that doesn't let you build them on the enemy path, making them useless.


The 2 damage towers vary by level, but my favorite combos were:

a) Mortar + Violator
best damage and range

• PROS: The most powerful build. Excellent range -- with perks and the right tower placement, Mortars can reach across 3 lanes! No perks needed in the Energy tree.
  
• CONS: Terrible on maps with air units. The Mortars can't hit them.

The Mortars' AOE devastates the cold hallway, and the Violators (whose damage increases exponentially each upgrade) devastate any lone stragglers that break through, no matter how fast or big. Remember to set the Violators' target mode to First.


b) Flamer + Flash
good all around, bonus interrupts and stun-locking

• PROS: The best all-around build. Good on maps where enemies cast lots of skills because Flash blankets your hallway with interrupts. No perks needed in the Physical tree.
  
• CONS: Mediocre damage compared to Mortar + Violator. Flash can't hit air units (though its splash still hurts them, I think).

The Flamer is the most useful standalone tower in the game (great range, damage, price, and utility), in my opinion. Flash's ministun (upgradeable with perks) makes your hallway even icier.


c) AntiAir + Flamer
excellent against air units

• PROS: EXCELLENT on maps with lots of air units - which in my opinion are by far the hardest maps.
  
• CONS: Anemic ground damage. Unless you time your pushback skills perfectly, expect some Armored Walkers to survive your icy hallway and plow through your stockpile of Laser Mines. Also, you have to divide your perks between the Physical and Energy trees.

The most fun. NOTE: The Gravity Shift skill is very useful with this build: it temporarily lets your AntiAirs light up a floating, stunned ground unit. (Just don't Force Push the floating unit....)

I almost always take:
(1) FORCE PUSH (pushback)
(2) REVERSE TIME (pushback)
(3) BLACK HOLE or GRAVITY SHIFT (pushback)
(4) LASER MINE (damage)

Pushback and damage, that's all you need. Mostly the pushback.

The only exception is when I take the top-tier Skills perk, and so I take Bomb, Mine, and/or Lightning to benefit from it. But this is rare. (More on that later.)

• Force Push: ALWAYS take. You can push the boss all the way to the beginning of your slow corridor, over and over again, every 7 seconds. Doubles as your main enemy skill interrupt.
  
  
• Reverse Time: I preferred this over Freeze or Teleport. Short cooldown, and I liked how easy it was to cluster enemies with it for more AOE damage.

• Mine / Laser Mine: Bread-and-butter damage skill. Excellent because you can stack up a huge minefield in the back of your maze while things are going well. Always have this skill on cooldown. Laser Mine is better (esp against air units) because it's more flexible and does more damage in my experience:
  - you get two Laser Mines for the same cooldown as one Mine
  - Laser Mines' huge AOE usually hits far more units - VERY good against flying enemies!
  - shorter cooldown = more flexible = Laser Mine doubles as an emergency damage button
  - EXCEPTION: if you have the top-tier Skills perk, take Mine, because it benefits from it and Laser Mine doesn't
  
  
• Bomb / Lightning: Complementary damage skill. Bomb is better because you can hit TONS of enemies clustered in your slow corridor...except on the Square map, where the mobs are spread across 4 launch points.

• Black Hole: Useful pushback when you need a low cooldown (eg maps where you need a lot of interrupts) and/or want to cluster your enemies for better AOE damage.
  
  
• Gravity Shift: Useful pushback for ground waves of maps with air levels: freeze the boss and let your army of AntiAirs destroy it. Low cooldown. Careful not to Force Push a floating enemy, as this will permanently transform it into an air unit.

My perks varied a lot. Again, you should probably skip this text guide -- the individual map guides (with screenshots) are more useful -- but I'll include it just in case.

ALWAYS take the first 3 points in:
- Physical (unless you have no physical towers)
- Energy (unless you have no energy towers)
- Effects (that 10% extra slow is ESSENTIAL to your icy hallway, and the slowing DPS (a) adds up when you consider how long bosses and heavies are trapped in your ice corridor, and (b) is nice against runners)

- The first wave is often the hardest because you can't build enough towers. On maps that require you to build a whole maze of tower bases, even one point here can really help.
- The cash bonus 3-point perk is often the most useful thing you can spend your last points on. It's basically a free tower every 2-3 waves.

- On levels like Complex where you really have to shove them back, maxing this tree is very useful. A 5-second cooldown Force Push is hilarious.
- The top-tier perk makes Bomb, Mine, and Lightning take away at least 5% of any enemy's MAX HP, including bosses. In late-game Survivor Mode, this is your ONLY source of damage. If you have this perk, this is the one time where I recommend taking two damage skills (Bomb and Mine) instead of just one.

- On some maps it's useless. On Crosshair it let my Mortars hit 3 lanes instead of 2. On Gallery it significantly increased the number of damage towers that could hit my slow corridor.
- Most of the time, I only put a spare point in range.

- The top-tier perk-skills (pierce armor, combustion, and money rain) were less useful than the skills I was already using.
- I didn't use damage boosts, so the bottom half of the Effects tree was nearly useless to my build.

This map was...easy. Mobs rarely survived the corridor, and never once reached the first teleporter. (Hence that stockpile of laser mines.)

Suggested improvements:
- I used damage pads instead of slow pads just to speed things up, but slow pads would have been better.
- Would have been smarter to build the corridor in the middle two rows and get the energy tower range perk, so the towers could shoot across 3 lanes instead of just 2.





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Important perks:
- 3 points in Energy
- 3 points in Slowing/Effects

That's all you really need. (Unless you're on Survivor mode.) Go wild.

Slow-lock and pushback are more important than damage on this map. Once the Kairos and Slow Pads were upgraded, it was extremely rare for a mob to leave the icy corridor (hence all those extra mines).

Low-cooldown skills help a lot, especially on early waves.



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Important perks:
- 5 points in Energy (extra range makes a BIG difference whether 4 towers or 8 towers are hitting any given mob - you don't have a lot of real estate here)
- 5-6 in Skills (low cooldowns important)
- 3 points in Slowing/Effects

Dodgers are a problem on this map: every bullet counts because their flight path to the goal is so short and they dodge so many bullets.

This is why those two disconnected slow pads in the front are CRUCIAL: air units fly right over them. Slowing dodgers by 50-80% vastly multiplies the number of hits they take, especially when you have a "quantity over quality" policy with your Anti-Air towers.

(I had trouble Force Pushing enemies back through my diagonal corridor. Might have worked better if the corridor is one straight line.)

Gravity Shift is exceptionally useful as a pushback, because all your Antiair towers will then proceed to light the poor sap up. (Just be careful not to Force Push a floating enemy, or it will be permanently classified as an air unit and walk right into your goal.)





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Important perks:
- 3 in physical damage
- 3 in energy damage
- 3 in economy (lots to build; makes early waves a lot easier)
- 3 in slowing/effects

Low cooldown skills mattered here. Adding that last Skills point made Bomb and Mine do a LOT of damage on low cooldowns, which was fun.

The mobs never once escaped the corridor, so my Deployers were useless. Violators probably would have been better, but I spent all my money upgrading my Mortars anyway so it didn't matter.

I experimented with Teleport instead of Reverse Time...I like Reverse Time better.

Adding a Kairos to the very beginning of the maze probably would have helped.





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Important perks:
- 3 in physical damage (I didn't have all my perk points yet; putting some into physical range would help)
- max Skills tree
- 3 in slowing/effects (as always)

This one was tricky, especially the early waves. Economy perks made a big difference.

The Lightning skill helped because there are a lot of dispersed Mutators charging skills all over the place.





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Important perks:
- 5 in physical (need that range)
- 3 in economy
- spare points in skills helped
- 3 in slowing/effects

This tower setup forces the mobs to pass your mortars three times. Set all damage towers to target mode First. The map got a lot easier mid-game, after I bought the Violators (set to target mode First).

Build Kairos before Slow Pads.

Black Hole was nice to help collect all the enemies that survived one pass by the towers into one nice big AOE-able bunch for the next pass.

A diagonal corridor is hard to Force Push enemies back through, but I don't see any alternative.





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Important perks:
- Physical range perk very important so Mortars shoot across 3 lanes. (Didn't have that extra point when I started. Must have leveled up.)
- Economy perks help those tough early waves, with so many tower bases to build.
- Slowing perks are as essential as ever.

Same exact setup as the previous map, except minor perk point tweaks.

Like the last map:
- the mobs pass your towers 3 times
- set all damage towers' target modes to First
- Black Hole is useful to cluster survivors up into a nice big AOE-able bunch (and doubles as a low-cooldown interrupt)





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Important perks:
- physical damage (don't really need range here)
- economy (at least 1 point...lots of building, early waves are the hardest)
- 3 in slowing/effects

I probably could've rolled the exact same setup as last time, but I like experimenting.

I put this map at the bottom because it's the least representative of my build. The black sheep of the family.

The first waves are hard because money is so tight. (Economy perks help.) On the first wave of heavies, expect a lot of desperate Force Push and Bomb/LaserMine adventures.

Your earliest towers should include:
- Flamers that can hit both sides (ie, the ones built right next to the gap)
- the back Kairos (slows mobs for the first Flamers to hit)

Exponentially easier once you get a L2 Kairos.

Deployers are uniquely useful here because they're 1) cheap 2) excellent damage against both straggler runners and all those pesky air units.

If this isn't working for you, try using Slow Fields instead of Deployers for some cheap, early slow-locking (as seen in every other level here).





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Important perks:
- 3 points in Physical, Energy, Economy, and Slowing

I didn't have all my perk points yet, but all these were essential. It's basically +damage, +slow, and +money (which I found necessary, given this map's shoestring budget).

Modifications for Survivor mode:
- In late-game Survivor...towers do almost zero damage. Your only hope (correct me if I'm wrong) is the maxHP%-based damage of the top-tier Skills perk, and casting the Bomb skill over and over and over and over again until they die. (I would have loved to include Mine or Lightning to increase my damage, but I found I needed all 3 of my other skills as pushbacks to keep the mobs inside my maze. Maybe it's possible though...?)
- This is hideously boring, but at least you're also farming that "kill 10,000 walkers" achievement.
- On the Gallery map, you can't make a corridor of Slow Pads. To compensate, use Flash towers to stunlock, enhancing Flash's ministun with the top-tier Effects perk.

Basic idea:
- Slow them down with Kairos + Flash's ministun
- Push them back with Teleport, Reverse Time, and Force Push (if you time it really well, you can keep them all stuck forever in the first long stretch)
- Bomb skill is your ONLY source of damage after...I forget...wave 40? 50? 60?
- putting a ton of Black Hole towers at the end of your maze will catch stragglers and hold them there for a loooooooong time (which is great for letting the cooldowns of your all-important skills reset)...at the cost of interfering with your skills sometimes
- do everything you can to keep them bunched in ONE Bomb-able group...even one stray runner means you have to cast Bomb another ~20 times just to kill it
- Penetrator tower hits so many enemies I couldn't resist using it...it was fun while its damage was greater than zero
- each wave takes 4-10 minutes...eventually I just suicided





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Important perks:
- Everything here is vital. You need the top-tier Effects perk (Flash's stunlock, vital component in your icy corridor), and you REALLY need the top-tier Skills perk, as it will quickly become your ONLY source of damage.
- Hilariously, Energy perks aren't all that necessary...those towers' damage becomes obsolete soon enough anyway. Throw whatever spare points you have in Energy.

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Caveat: Those obnoxious first two waves:

By "obnoxious" I mean "the most fun."

Took me a while to figure out how to spend my first dollars to kill those first few mobs. Here's what I finally came up with, I believe it's self-explanatory: