The Keep: Zombie Horde

Between waves, the player can spend the earned money (rewarded for each saved survivor) for new weapons, more ammo or more tower floors. Every other wave is a bonus round during which as many zombies as possible have to be killed with a machine gun: the more deaths, the more survivors appear in the next wave. In contrast to other wave-based shooters, the waves last relatively long (about five minutes) and even the standard pistol has limited ammo. The alternative is a knife, but it is only useful against zombies which manage to enter the tower and attack the player directly.