When nighttime draws near and aggressive mobs start to spawn, a few villagers will run into their houses. Some villagers will dwell around in the village until zombies will chase them around the village. Villagers will run into the nearest house, even if it is already housed by other villagers and slam the door. Zombies will pound on the door in an effort to break in. If a player is playing on a difficulty of "hard", or is using a hardcore map, zombies will eventually break in, attacking villagers as they flee. In most occasions, a swarm of zombies will flock around one door, leaving other houses open for villagers to run into.
This newly added event has also led to the creation of a new artificial mob: the Iron Golem. Iron Golems will only spawn in a village inhabited by sixteen or more villagers, or they can be made by the player (see the Iron Golem page for their building recipe). They will immediately attack any zombies present.
Notch intended to add this event as a possible feature during the early stages of Minecraft. The idea of sieges, with the usage of NPC villages, was added in the 12w07b snapshot for 1.2.
Iron Golems were added in the 12w08a snapshot a week later.
As of 12w32a, zombies are now potentially stronger because of the addition to mob armor and mob weaponry. A new mob, known as the Zombie Villager, can also be spawned as a result of a zombie killing a villager.