There's a castle made entirely of salt
The Hallstatt Salt Mine Castle in Austria is a unique structure built into the side of a mountain, but it's not made of stone or brick - it's entirely composed of salt. The castle was built in 1972 by a miner as a way to preserve the history of salt mining in the area. The castle has several rooms and even a chapel, all carved into the salt deposits. It's a fascinating example of a castle built from a surprising and ephemeral material.