The Labyrinth is a one-level generated structure added by Project Zulu that is found in the Overworld and houses both treasure and monsters. It will only ever have a staircase opening on the surface where it will go down into a maze where Mimics, Minotaurs, Haunted Armor, and Vanilla Minecraft mobs lurk.
Spider Webs that can make navigation a challenge litter the Labyrinth.
There are areas where the wall is entirely cracked stone bricks. In this instance, the wall will likely be hiding a concealed room possibly containing treasure and Monster Spawners.