Where I find these sorts of concerns curious is that in a game based on make-believe, we can just make up how the dungeon got there and what it was once used for (or anything else about it), so isn't this sort of admitting to a failure of imagination?
Not at all - rather, I need to exercise my imagination to justify in my own head and understanding. Because I guarantee my players will ask backstory questions about it (
especially if I didn't work it out!). My players are almost all "engineers" by background and personality, they want to know how things work.
And in turn, my campaigns are presented as "living" worlds, where things happen whether they are present to see them or not. They came back from a three-week trip (to that tower with the rift, in this case) to find the town they had left from had been captured by goblins in the meantime. Cue multiple sessions of guerrilla warfare culminating in a grand battle to recapture the town! But while they were busy doing that, political maneuvering farther into the kingdom embroiled two of the PCs' fathers in an international conflict (hence their interest in rescuing this noble, to get involved in
that issue). The PCs' efforts earlier in the campaign led to some large troop movements, some losses, and thus when they came back to their home base, they heard about a massive military recruiting effort by the local Baron. Is he gathering troops to restock after losses, or to get involved in the growing political turmoil?
Then my players take things further, and wonder if that tower they cleared out to the north can be used by the Baron as a military post? (Yes, the PCs worked out how to shut down the power source for the automated defenses.) What about that village of undead they cleared, the one with fresh water... is it safe enough for a logistics staging base? (No, the PCs worked out how the goblin army uses it - indeed, used it to take that other town recently.)
They put together rumors from three places, and their understanding of the economy of the local Barony, to figure out the missing "traveler" was actually a key noble in the area, and that going to rescue him would be good. And the guide the noble hired was the "right guy for the job" because of his burnberry harvesting area in the swamps... and the guide the
party hired, his rival, was clearly the second-best guy.
So yeah, in this game about make-believe and pretend, I need to understand in my own head why this "dungeon" (in any form) is there for the PCs to explore. And why it hasn't been cleared out already - or how it changed/recovered from being cleared out. And where the bathrooms are....