Minecraft wiki entry for Legacy_village_mechanics, associated text: At night, there is a chance that a zombie siege might occur. This is when a large number of zombies spawn in or near a village, attacking what villagers they can reach, crowding around and pounding on the doors of those they can't. On hard or hardcore mode, they can actually break down the wooden doors of the villagers' homes (this is true of all zombies, not just during sieges.) A zombie siege requires a village of at least 10 houses and at least 20 villagers.