While an excellent question, it could simply be a dungeon crawl game. FYI, those are perfectly valid even if you and many other people don't believe it to be so.
OP, you have to decide first, what you are trying to do. Is it a dungeon crawl game or something else? It shouldn't take more than a few seconds to decide what this place is and why it is available to the PCs. Then you start breaking down the details of the place. You might want to come up with goals or not for this process. You might consider who, besides the party, knows about this place. Say the local lord knows about it, but hasn't dealt with the baddies inside for some "insert reason". Perhaps the lord doesn't know for some "insert reason".