Unlimited cash.
The bad guys are coming after your base!! They are doing their best to get into Area 51 (The name of your base) and you have to build towers to prevent the bad guys from getting there.
Every bad guy that you kill, you get an amount of money depending on what wave number you are on. When you get this money, you can use it to build new towers or upgrade your existing towers to gain additional powers and enhancements.
1. The Modifier Turrets
These turrets have no direct firepower themselvs. They only increase the abilities of other turrets to give them extra damage, speed, or range. You need to strategically place them to win.
2. Exchange Turrets
These turrets are kinda just like the modifier turrets in the fact that they give you a bonus to speed, damage, and range. Except these will increase one statistic and decrease another. You can find useses for these, a good example is when you do not need any range but you need more damage because the turret is already in the range of everything that is going to hit.
3. How to use Combos
Combos are the greatest thing in this game, they take a lot of time to build up but they are very worth it. Infact you will not be able to get very far into the game w/o them.
All turrets in the group must be next to eachother, at least less than one turrets-width apart
All turrets in the combo range must have the damage upgraded to the maximum level, I recomend you leave the range and speed at their normal levels.