Turret is a FPS / tower defense game. The enemies are spawning in waves and march toward an obelisk that you must protect. You can shoot these enemies, but the most powerful weapon is the ability to build automated turrets.
Each enemy yields 1 credit when killed, and each credit can be spent to either build a new turret or upgrade a turret’s health, range, or damage. The enemies scale up quickly in power, so it’s essentially a score attack game.