The Cathar castles were fortifications and buildings built for lots of different reasons. A lot of them were already built before the Cathar Crusade, but were used by the Cathars as protection. A lot of the original castles were left to degrade, or were replaced by better fortifications. Some of the castles that are called Cathar castles today were not even around during the crusade! They were built in the same region to protect France from Spain.