I totally didn't realise that we were most of the way through the One-Page Dungeon contest, so I've started scribbling some ideas down. An ancient and degraded pit carved into the ground long before anyone can remember, there is a long winding spiral pathway from top to bottom (and a few settlements that see just enough traffic from travelling adventurers to remain viable trading posts). The walls are riddled with mines and caves, and journeying into one of them may lead you through a labyrinth and out through another. Mystic energy and swirling vortexes mean that travellers generally cannot fly down the shaft, and even if they do risk the winds, there are nests of harpies and other dangerous creatures also willing to take that risk for a quick feed. Going into the caves can be just as dangerous.