Dyson's "The Lost Ossuary" is a small dimensional rift beneath the Lobachevsky Church. Cut out of the stone beneath the church as a set of crypts and ossuaries, the Lost Ossuary displays bizarre geometries to those who would try to map it out.
Good example of a Gnome or Changeling Treehouse. Gnomes often build closer to the ground and underground. Changelings will build taller trees and make it hard for anyone other than a shapeshifting Changeling to reach their abodes.