Then they thump inside it, and I realize how little detail I had given. Especially given that our characters will, of course, be obsessively checking every detail to figure out where they are. But are the stones great thumping blocks? A jumble of rocks gathered up in broken form (perhaps from a river bed) and mortared together? How dark is the stone?
Hmm. Maybe I can make it parallel, in form, a labyrinth they already know about. That would add to the creepiness. As if being magically abducted and having no notion where you are or who your kidnappers are is not creepy enough.