Dungeon is a term that widely refers to a variety of creature-made or naturally occurring complex of rooms and corridors - often underground, or sprawling across or under large buildings like castles, temples or similar. The term is usually used for this latter kind only if they’ve been abandoned for their original purpose, such as in ruins, as “a dungeon” is usually somewhere occupied by dangerous monsters, bandit or barbarian-like people, as well as various Haunts and Traps set up or left behind there. It’s a prime destination for Adventurers to try to clear out or find loot within.