Cité de Carcassonne audio guide,
five stops, free.

You are on the east side of the Cité, looking west at the two towers of the Porte Narbonnaise, with the bridge and the dry moat in front of you. This is the main gate, and it faces this way for a reason. On the east the hill slopes gently, which made it easy to bring goods up, and just as easy for an army. So the heaviest defences in the whole fortress were put here.

Look at the shape of the towers. They are not simply round. Each one is drawn out towards you into a spur, so that shots aimed at it glance aside instead of striking square. Each tower is thirty metres high. On the second floor a large guard room joins the two, and they doubled as storerooms, one with a cistern and the other with a cellar, so the men inside could hold out through a long siege on their own. Before it was called the Narbonnaise it was the Porte de Pressan, and it was designed as an independent fortification. The passage behind the arch was shut by two portcullises, with a murder-hole above for anyone caught between them.

It went up around 1280, when Philip the Third and Philip the Fourth decided to give Carcassonne new walls, and the message pointed south. From here the foothills of the Pyrenees are in view, and beyond them lay the kingdom of Aragon, the rival France was guarding against.

Above the gateway, between the towers, a niche with a three-lobed top holds a statue of the Virgin. Now look down by the bridge for a stone bust of a woman with a strange, smiling face and the words CARCAS SVM, I am Carcas, at its foot. It is a copy; the sixteenth-century original is kept inside the château. The sculptor used an old medieval keystone for the head, and the daisies on her sleeves may be a compliment to Marguerite d'Angoulême, sister of King Francis the First, who passed through in 1538.

The legend says Lady Carcas held the city against Charlemagne for five years, fed the last pig on the last sack of wheat and threw it from the highest tower, so the Franks decided the town was overflowing with food and left. She rang every bell, a soldier shouted Carcas sonne, Carcas is ringing, and the city had its name. The history is plainer. The Franks who took Carcassonne from the Saracens in 759 were led by Pepin the Short, and his son Charlemagne was seventeen.

A confession about the bridge, before you set foot on it. The drawbridge you are about to walk over was added by Viollet-le-Duc in the nineteenth century, when he rebuilt the gate's crenellations and roof in 1859 and 1860. The original gate never had one.

Back inside the walls, walk west through the Cité until the ground opens out in front of the château. Stand before its barbican, with the bridge and the moat ahead of you, and take in what you are facing: a fortress inside a fortress.

It did not begin like this. In 1067 the Trencavel family received the viscounty of Carcassonne, and around 1130 they built a palace here, a palatium, standing on the old Roman wall on the west side, where the slope falls away most steeply. It was a residence, and little more than a palisade separated it from the town. When the crusade against the Cathars reached Carcassonne in 1209, crenellations were added to its roofs to turn them into firing posts. Then the city passed to the king, and between 1228 and 1239 the palace was rebuilt as the fortress in front of you: stone walls in place of the palisade, a moat running all the way round to the inner wall, a barbican before the entrance, and a gate between two towers with machicolation, a portcullis and heavy doors. The bridge was a fixed span, then a pivoting section, then a drawbridge worked by counterweights. The walls protected the king's seneschal not only from invaders but from the people of Carcassonne, whose loyalty to the crown was not yet secure.

Now look at the wooden galleries along the top of the wall and around the tower on the right. They are hoardings: timber boxes hung out over the drop, so that a crossbowman could shoot straight down at the foot of the wall and into the lices. Only a handful of medieval hoardings survive anywhere, because timber rots. These were reconstructed under Viollet-le-Duc, and they make Carcassonne one of the few places where you can see a castle wall dressed for a siege.

The roofs are the other quarrel. Viollet-le-Duc capped the towers with steep cones of slate, as castles were roofed in northern France, and was attacked for it, because this is a country of low roofs in Roman tile. His answer was that the crusaders who took the region came from the north and may have brought their own builders, and that he had found fragments of slate on the site. Both kinds of roof survive in the Cité today, and from the ramparts you can pick them out.

He did not live to see the château done. He died in 1879, and work here only began in 1889, under his pupil Paul Boeswillwald and then Henri Nodet. Before that, in the nineteenth century, the building had served as a prison.

From the château the route climbs onto the western walls, the side the operator's own guide to the ramparts turns you towards. Face west, out over the parapet. The ground falls steeply below you, and that is why the château was built against this wall: on this side the slope did half the defending.

Start with what is closest. The walls below zigzag down the hill towards the Porte d'Aude, the west gate, which faces the river. It was designed as a trap. High arcades hide false doors that lead nowhere, and the passages climb in switchbacks with landings, so attackers who forced their way in were held in one pen after another and shot at from every side. The gateway arch itself is Visigothic, round and banded with brick; the heavy projecting bay above it is Viollet-le-Duc's, and historians count it among his mistakes. The same gate has played itself on film, in Les Visiteurs, in Robin Hood, Prince of Thieves and in Le Corniaud.

The towers along this front are the old ones. Look for the horseshoe shape, curved towards the outside and flat towards the town: the Roman plan, rebuilt by the Visigoths and again by the French kings. The wall works started on this side, on the west-south-west stretch of the inner wall, in 1855, and work to make this western circuit safe for visitors began in 2014.

Now lift your eyes across the Aude. The town on the far bank is the bastide Saint-Louis, the lower town, and it exists because of a failed revolt. In 1240 Raimond Trencavel tried to win the city back with help from the suburbs outside the walls. When he failed, Louis the Ninth had the traitors' houses razed and sent their owners across the river to begin again. Two towns, one on each bank, from a single punishment.

Then look to the horizon on your left, to the south. The hills are the Corbières, and on a clear day the Pyrenees stand behind them. For centuries that was the direction that mattered: Aragon lay beyond, and everything you have walked through today was built to face it. In 1659 the Treaty of the Pyrenees moved the border south, the fortress lost its purpose, and in 1804 Napoleon had it struck off the list of strongholds. In 1849 the government decided to pull the walls down. Local people fought the decision, led by Jean-Pierre Cros-Mayrevieille and Prosper Mérimée, and it was reversed. The restoration ran from 1853 to 1911, and what it saved is fifty-two towers on three kilometres of double wall, a World Heritage Site since 1997.