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.