Catacombs
Any time Men lay their dead to rest, others may find reasons to disturb their slumber. Whether because the dead are often buried with their riches (or their secrets), or because darker, fouler rituals require materials only found amongst the decay of the grave, there is often little peace for the passed-on.
While catacombs can sometimes be discovered unplundered and sealed to protect their fallen wards, they are equally as likely to be defiled and inhabited by bandits, foul beasts, or more sinister enemies who require proximity to the dead to do their dark work. Should you ever find an occasion to pay a visit to the sleepers in their ivory halls, be certain your lantern is full and your need is great.
BACKSTORY
Catacombs exist from the Age of Kings onward.
BUILDING
Catacombs are very much like more traditional role-playing game dungeons, with a few central chambers and many twisty, windy passages, all decorated with the interred dead and their tokens.
For most “classical” dungeon crawls, it’s safe to consider them in the Catacomb category - some will be defiled and repurposed by monsters or other villains, some will have remained unplundered, but their original guardians and dangers still await intruders. This allows you, as the worldbuilder, maximum latitude in creating labyrinthine maps and wondrous treasures without breaking the realism of your setting or the logic of your characters.