He wants to take revenge on Simon de Montfort who dispossessed his father 31 years before. Carcassonne now belongs to the Kings of France. But the siege is long and difficult, and despite the help of the inhabitants of Carcassonne's suburbs, who are still faithful to their former lord Trencavel, it is impossible to take the city.
Trencavel eventually breaks camp, he resigns himself to the fact that he will never be viscount of Carcassonne, and leaves to live in exile.
The inhabitants of the two suburbs St Michel and St Vincent are punished by the representatives of the royal power, they are all exiled and their houses are razed. St Michel and St Vincent disappear forever.
Seven years later, king Saint Louis authorizes the exiled inhabitants to come back. But he distrusts them, the region is still rebellious and hostile to the Frenchmen, so the "Carcassonnais" have to settle down on the other side of the Aude river, as far as possible from the fortress. This new settlement on the left bank of the Aude will become the Lower Town, also called Bastide Saint Louis.
After agreement of the four owners of the ground, the bishop, the chapter, the king and the order of the Temple, the works begin in 1248. They last until 1262, when the first inhabitants come and settle in their new houses. Faithful to the memory of the two former boroughs, the inhabitants clustered in two parishes, Saint Michel to the south and Saint Vincent to the north. These two churches still exist today !
The Lower Town looks like Manhattan (in much smaller proportions !!!) with its grid lay-out, its streets crossing each other in right angle. The blocks of houses are called "carrons". There were 65 of them before the Revolution. From East to West the streets were called "rues", and from North to South "traverses", nobody could get lost !
The Lower Town, Bastide Saint Louis
At the same time the Cité undergoes important modifications : construction of the second surrounding walls, the Barbican of the Counts' Castle, and construction of the main towers. The Tour du Trésau (Treasure Tower), Tour de la Vade (Vade Tower), Tour de l'Evêque (Bishop's Tower) and Tour St Nazaire (St Nazaire Tower) all date back from this period. The St Nazaire Basilica is partly demolished, the romanesque choir is replaced by a new one, built in the new style coming from northern France, the Gothic style (1269).
This is the origin of Carcassonne's current aspect.