The first signs of settlement in this region have been dated to about 3500 BC, but the hill site of Carsac—a Celtic place-name that has been retained at other sites in the south—became an important trading place in the sixth century BC. The Volcae Tectosages fortified it and made it into an oppidum, a hill fort, which is when it was named "Carsac".[1][2]

The folk etymology—involving a châtelaine named Lady Carcas, a ruse ending a siege, and the joyous ringing of bells ("Carcas sona")—though memorialized in a neo-Gothic sculpture of Mme. Carcas on a column near the Narbonne Gate, is of modern invention.[citation needed] The name can be derived as an augmentative of the name Carcas.

 Miniature depicting Cathars being expelled from Carcassonne in 1209

Carcassonne became strategically identified when the Romans fortified the hilltop around 100 BC and eventually made it the colonia of Julia Carsaco, later Carcaso, later Carcasum (by the process of swapping consonants known as metathesis). The main part of the lower courses of the northern ramparts dates from Gallo-Roman times.[1][2] In 462 the Romans officially ceded Septimania to the Visigothic king Theodoric II who had held Carcassonne since 453. He built more fortifications at Carcassonne, which was a frontier post on the northern marches.[clarification needed] Traces of them still stand.

Theodoric is thought to have begun the predecessor of the basilica that is now dedicated to Saint Nazaire. In 508 the Visigoths successfully foiled attacks by the Frankish king Clovis I. In Francia, the Arab and Berber Muslim forces invaded the region of Septimania in 719 and deposed the local Visigothic Kingdom in 720;[3][4] after the Frankish conquest of Narbonne in 759, the Muslim Arabs and Berbers were defeated by the Christian Franks and retreated to Andalusia after 40 years of occupation, and the Carolingian king Pepin the Short came up reinforced.[3][4]

A medieval fiefdom, the county of Carcassonne, controlled the city and its environs. It was often united with the county of Razès. The origins of Carcassonne as a county probably lie in local representatives of the Visigoths, but the first count known by name is Bello of the time of Charlemagne. Bello founded a dynasty, the Bellonids, which would rule many honores in Septimania and Catalonia for three centuries. In 1067, Carcassonne became the property of Raimond-Bernard Trencavel, viscount of Albi and Nîmes, through his marriage with Ermengard, sister of the last count of Carcassonne. In the following centuries, the Trencavel family allied in succession with either the counts of Barcelona or of Toulouse. They built the Château Comtal and the Basilica of Saints Nazarius and Celsus. In 1096, Pope Urban II blessed the foundation stones of the new cathedral.

Carcassonne became famous for its role in the Albigensian Crusades when the city was a stronghold of Occitan Cathars. In August 1209 the crusading army of the Papal Legate, abbot Arnaud Amalric, forced its citizens to surrender. Viscount Raymond-Roger de Trencavel was imprisoned while negotiating his city's surrender and died in mysterious circumstances three months later in his dungeon. The people of Carcassonne were allowed to leave—in effect, expelled from their city with nothing more than the shirts on their backs. Simon de Montfort was appointed the new viscount and added to the fortifications.

 Expansion of the Frankish Empire:
Blue = realm of Pepin the Short in 758;
Orange = expansion under Charlemagne until 814;
Yellow = Marches and dependencies;
Red = Papal States.

In 1240, Trencavel's son tried to reconquer his old domain but in vain. The city submitted to the rule of the kingdom of France in 1247. Carcassonne became a border fortress between France and the Crown of Aragon under the 1258 Treaty of Corbeil. King Louis IX founded the new part of the town across the river. He and his successor Philip III built the outer ramparts. Contemporary opinion still considered the fortress impregnable. During the Hundred Years' War, Edward the Black Prince failed to take the city in 1355, although his troops destroyed the lower town.[5]

In 1659, the Treaty of the Pyrenees transferred the border province of Roussillon to France, and Carcassonne's military significance was reduced. Its fortifications were abandoned and the city became mainly an economic center of the woollen textile industry, for which a 1723 source quoted by Fernand Braudel found it "the manufacturing center of Languedoc".[6] It remained so until the Ottoman market collapsed at the end of the eighteenth century, then reverted to a country town.[7]