Where things spawn.
Spiders and Cave spiders usually spawn in slots in abandoned mineshafts that are covered in cobwebs, skeletons spawn in cobblestone dungeon cells, zombies spawn in mossy cobblestone, as well as spiders, creepers don't have a dungeon cell, but they are very dangerous with zombies while in a dungeon cell, because the zombies can corner you, while the creeper will be the bane of your existince.