Minecraft wiki entry for Village_mechanics, associated text: At midnight, there is a 10% 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 (this is true of all zombies, not just during sieges). They'll attempt to beat them down on other difficulties, but not succeed. A zombie siege requires a village of at least 20 villagers and at least 10 beds.