Keep the creeps out of Area 51 by placing turrets on the map. The timer in the lower right-hand corner shows when the next wave will invade. This can be sped up by clicking on the timer or pressing space bar. You can use the money you earn from killing creeps to buy and upgrade turrets. The basic turrets have three upgradable attributes; damage, range and rate. More money is required each time you upgrade a turret's attribute. Combos can be triggered by grouping certain turrets together and upgrading their damage to the maximum level.

Red Turret Combos:
+ = Laser Rockets
+ + = Poison Gas Rockets
+ + = Nuke

Green Turret Combos:
+ + = BFG
+ + + = Rocket BFG

Blue Turret Combos:
+ = Land Mine
+ = Cluster Bomb
+ + = Sparky Cluster Bomb
+ + = Big Land Mine
+ + = Sparky Land Mine
+ + = Big Cluster Bomb
+ + = Laser Cluster Bomb
+ + + = Big Laser Cluster Bomb
+ + + = Big Cluster Bomb Land Mine
+ + + + = Big Laser Cluster Bomb Land Mine

Yellow Turret Combos:
+ + + = Black Hole