I think it would be a good idea to make this transition impassable for monsters. Earlier the hag mother chased me right out, and while I though she was far behind me, some other people ran into her wandering Ranzington. Both for the sake of being able to flee from such a monster at lower levels, and because the hags that live under the city would know they'd get lynched if they surfaced, I think it should be barred for them to exit.