The walled city of Carcassone was abandoned after the Albigensian crusade. Brief historical recap. Northern France declared war on Southern France, damn heretics, killed them, and took their stuff. What this means for us is that the city was left largely intact because people didn't build over bits in subsequent centuries.
In the 1830’s it was restored. The architect decided that since all the medieval pictures of castles (from the north of France where there is a lot of slate) have blue roofs, he’d rebuild the towers with blue roofs.
We walked all the way around the city on the city wall. This tower was built by the Romans who knew how to build.