A dungeon is a place left behind by a previous civilization, race, or group that wanted to leave stuff behind for some reason. some contain loot, some contain ancient evils, and some are coming of age trials. A dungeon typically contains traps, monsters, and of course the reason people don't just block them off forever, loot.
While [Mages] study which empire had built a dungeon and what their composition is, Adventurers have thought of a simple system to categories them. A dungeon is either intended for containment, future generations, or as an act of vengeance.
Those dungeons are meant to contain something. It could be a threat, a natural resource too valuable to let someone else get ahold of, and so on. One such dungeon, for instance, had been built solely to combat Vampires, therefore used mostly light-based traps and spells.
Dungeons built with traps that test people entering. For example, an empire is about collapse. Before it comes to that, however, they desire to leave something behind. Not that they would like their valuable artifacts to go into the hands of just anybody, though; only those that have proved themselves should receive them. As a result, they built one only the best can enter.
If an empire or civilization is upset at the thought of future generations to take what was once in their possession, they build a dungeon to seal away their most valuable treasures. Then, they fill it with the nastiest traps and monster eggs. So if adventurers were to go into them, they would be attacked by an army of monsters that attempt to eat them. Such dungeons are designed to kill whoever dared to enter it.
- Dungeons traps claim more lives than monsters most of the time.