With its 3 kilometres' surrounding wall flanked by 40 towers, Carcassonne is the vastest fortress in Europe! Successively, Romans, Visigoths, Saracens besieged the city. In August 1209, Simon de Montfort and his army besieged the city during the Crusade against Cathars. Why? Because Carcassonne was a real Cathar bastion.
Fortunately, they had restored the old wall raised by Visigoths in 1130 and raised a castle. But this was not enough... 450 inhabitants were murdered because they didn't want to recant their Cathar faith... Then, our city fell to the Crown.
Carcassonne was, in the 13th century, surrounded by 2 walls flanked by several towers, reinforced by king saint Louis and later his son Philip the Bold. You'll recognize those 2 ones: towers raised in the reign of saint Louis are round shape, towers built by Philip the Bold are spur shape (like the Narbonnaise gate).
Before its restoration by the famous architect Viollet-le-Duc in the 19th century, Carcassonne was part of the defensive line defending the frontier between France and Aragon, with its "5 sons" (Aguilar, Quéribus, Peyreperyuse, Puilaurens and Termes castles). Well, until the signing of treaty of Pyrenees in 1659...