Zombie siege

Sieges are an upcoming feature in Minecraft. The first part of the project was added in 12w07a. Currently, zombies have a chance of coming in to the village in packs and attacking the villagers at night. Usually, a whole pack will concentrate on one door or villager. Villager breeding will keep the population up. As of 12w08a, villagers have Iron Golems to defend them.

Trivia

 * When a zombie is attacking a villager or door, it usually cannot detect you.
 * Zombies also attack baby villagers.