First, if you are one of my players . . . scat! Reading my blog will just make your gaming less fun.
Okay, the cool thing about having your megadungeon be a huge cylindrical hole is you have this great accessibility. Players can go as deep as they can manage, especially as they become more powerful.
But . . . what about all that wasted space? The Maw is ~200' across. And at its great depth there is a lot of unused volume there. Seems like a waste.
Empty volume meet invisible tower. Ha ha. Isn't it perfect? I just thought of it today. Of course it is a thin graceful tower, so to watch birds flying or hirelings falling to their deaths you wouldn't notice it. But it is there.
So what features should an invisible tower have? I'm thinking lattice like terraces, so unsuspecting adventurers get a quick trip to the bottom of the Maw. But what else?