Zombie Defense is a minigame in Crystal Story. It functions like a Match 3 puzzle, requiring you to match tiles to deal damage to a Zombie King, who sends zombies towards you. Damage affects the front-most zombie, then the Zombie King, depending on which is closer.
There are four modes: Easy, Normal, Hard, and Expert. The game ends when the Zombie King is defeated or when any zombie reaches you, whichever comes first.