From the release:
Catacombs is Finger and Toe's baseplate-based system for creating extensive underworld crypts, catacombs, and hidden temples beneath the sands. Using the base-plates, walls, doors, and props, the system relies on restickable glue for its enormous flexibility and replayability.
This set was inspired by the Catacombs of San Sebastiano outside Rome and Kom el Shoqafa beneath Alexandria and has the parts to create Roman or Egyptian underworld complexes from the time of Cleopatra, although, your setting is limited only by your imagination, scissors, and glue.
When you’re done playing, strike the set. Pull off the flat walls and doors and stack them with the base plates. The other props (pillars, doors, connectors, etc) take up very little room.