• Make sure to upgrade slots under turrets, it increases turret's damage.
  
• General slot upgrading rule:
  Do not upgrade unless it's cheaper than upgrading the actual unit that you will place there. Example:
  
  • Commando unit's 1st upgrade cost $2000, but to upgrade the slot that you'll put him for 99% of time is only $500.
    
  • Slots under the Sheriff shouldn't be upgraded unless you maxed out your unit upgrade.

Sheriff - selected unit name
HP - how much damage can a unit take before it dies.
Damage - how much damage per shot a unit does. not a DPS meter.
Ratio - how many shots per minute a unit does
Range - how far a unit can shoot.(See upgrades section)

Get Marines as soon as possible, they have longer range than Sheriff with an automatic riffles. Can be upgraded with grenade launcher which is kinda like a free Grenade every few seconds fired by the unit.
Turrets are very helpful in the early game to deal with addons(Walkers), so that your main units can focus down the other type of zombies. TURRETS DO NOT FOCUS BIRDS or CRAWLERS.

Usually around round 15-20 is a good idea.
Reason? Money
(Usually by round 20 you should have maxed all neccesary units to farm $ for the next 5-10waves.)

The reason behind is that the more cash you got on hand, the less you gain per zombie killed and the rounds get harder and harder, so if you lose a unit, it might take you a whole new round to replace it(depends how expensive it was obviously).
Early on you'll be receiving more than $20 per zombie, while later on it will drop below $4. It all depends on how hard you find the map and how much you want to grind it if you find it easy. Once you reach rounds 20+ you only get like 400-600$ per whole wave cause it drops to 3$ per zombie.



Soon to come.

This will cover the general idea of unit placement in maps

How to farm $Cash.
I found this map easiest to farm, reason probably being is that you only have access to Sheriff, you can get like $20k early on in less than 10-15min. Here is the strat behind it:

V1 Basic information added