Minecraft wiki entry for Legacy_village_mechanics, associated text: Zombies in sieges ignore light levels and the 24-block minimum distance from the player, but other than that, behave absolutely normal (i.e., they do not spawn on glowstone or any other transparent or half block, need a 2×1×1 minimum space, etc.). They can also spawn INSIDE doors, making Iron Golem farms difficult to build on hard, as zombies beat down doors on hard (they'll attempt to beat them down on other difficulties, but not succeed). However, zombies won't spawn 128 blocks away from you, even though the siege is technically happening in the game code, so you're safe if you build it high up. You can make houses virtually zombie-proof simply by taking out one ground block from directly in front of the door, and, if necessary, rehanging the door such that the outside is "smooth," i.e. the door's position on its ground block runs consistent with the outside wall. This is because zombies can only break the top half of a door, and if they have to jump, they can never get through.