The fortified castle inside the Cité, built in the 12th century as the residence of the Counts of Carcassonne and later converted into a citadel within the citadel — a last refuge if the outer walls fell. The Château Comtal is the one part of the Cité that requires a ticket and a guided or audio-guided tour; admission also covers the rampart walk along the inner walls, which offers the best views over the lists and the outer fortifications.
Inside, the museum covers the history of the Cité from pre-Roman times through the Albigensian Crusade of 1209 — which saw Carcassonne surrender to the crusading army within two weeks — to Viollet-le-Duc's 19th-century restoration. The barbican gate protecting the castle entrance is one of the most photogenic parts of the entire fortification. If you only pay for one thing in Carcassonne, this is it: the rampart walk gives a completely different perspective on the scale of the defences.