This is a question that comes up perennially, it seems, especially (but not exclusively) among newer players to the game. They can’t seem to fathom why dungeons exist, and in the sort of realistic-fantasy world they prefer, they simply cannot suspend their disbelief long enough to enjoy a proper dungeon-crawl type adventure.
More’s the pity for them, I say.
But of course even in a realistic-fantasy milieu that demands logical explanations for everything (even the existence of flying fire-breathing dragons and demons that are only harmed by +1 magical swords), there are explanations that can be derived for the existence of extensive dungeon complexes, particularly those beneath castles or the ruins thereof.
There are, in fact, a number of different justifications for the existence of large dungeon complexes.
- An insane god did it. This is the quintessential scenario, hearkening back to the original Greyhawk campaign. Zagyg, or whomever, set things up for his own amusement, to test adventurers, or whatever.
- It’s abandoned and repurposed. This is something one sees less often, but it’s still not uncommon. An underground labyrinth that was built for some other purpose, such as an extensive mining complex, city for underground-dwelling peoples such as dwarves, etc.
- It’s a natural feature. Especially in karst topography, a series of natural caverns and caves is perfectly normal. It’s no big stretch to say that in a fantasy setting, such features could be exaggerated, so that cave systems like Carlsbad Caverns are much more common. This is the basic setup of the Underdark in Greyhawk and the Forgotten Realms.
- It’s a defensive feature. This, to me, is the one that most people overlook. In a world where tunneling creatures (dwarves, kobolds, elementals, etc.) exist, in a siege situation a castle could be easily undermined and thus infiltrated. This was, in fact, an historical siege tactic, but given the existence of magic, it seems that it might be a more common tactic. Putting an extensive labyrinth of twisty corridors designed to confuse invaders, sprinkled with traps, makes perfect sense if one needs to defend against tunneling dwarves or kobolds. Especially when the defenders will know the terrain extremely well. Once the castle is abandoned, this turns into the second choice in the list, as monsters and other things move in, set up lairs, drop their own treasures, etc.
I’m sure there are others, but I think the point is made. Large dungeons are by no means unrealistic, especially in a fantasy setting where magical creatures and spells make underground movement much more plausible than it would have been historically.