Viollet-le-Duc carried out substantial restorations, adding crenellations to the walls and giving the towers slate roofs. The use of slates has been strongly criticized, however, and restorers are replacing them with clay ridge tiles.

The Cité de Carcassonne is a medieval fortified architectural group located in the French city of Carcassonne. It is located on the right bank of the Aude river, on the hill, in the southeast part of the actual modern city. It was founded in the Gallo-Roman period. It derives its reputation from double surrounding walls (3 km in length) with 52 towers. The town has about 2,500 years of history: it has seen the Romans, Visigoths, Saracens and Crusaders. In the beginning of its history it was a Gaulish settlement. In the 3rd century CE, the Romans decided to transform it into a fortified town. The town was finally annexed to the kingdom of France in 1247. It was a strong French frontier between France and the Kingdom of Aragon. In 1849 the French government decided that the city fortifications should be demolished. Prosper Mérimée advocated saving them; Viollet-le-Duc worked on the restoration continued by his pupil, Paul Boeswillwald, and later by the architect Nodet.