Similar to tower defense style games, the likes of which you have probably played at least one of at some point. Use your mouse and click to destroy any and all incoming enemies that come towards your base. Do whatever it takes to protect your turret. Find upgrades and use the space bar to fire off bombs. Kill off enough foes and that wave will end, moving you on towards the next level. Naturally, at higher levels the waves last longer, contain more foes, and your enemies will be more aggressive. Keep at it though, and slaughter them into a delightful red mist as you gun them down with the giant laser cannon of your turret.