From the introduction:
The Halls Untoward are half a mile below ground. Originally they were a small way station along a very long underground road.
Over time, the halls accumulated shrines, small communities, workshops, temples, mad wizards, and strange travelers. Each carved a new space from the rock, took up an abandoned space and made it their own, or pushed out the occupants of an existing area.
The halls are divided into two halves, which are now only connected only by secret doors. The way through from west to east has been forgotten, and a century of abandonment has set it wandering along its own course. Pretty much anything could be down there.