The destiny of the castle and its occupants changed in the 13th century
Royal power conquered the Languedoc, an area hitherto dominated by the Count of Toulouse and his vassals. Sommières formed the gateway of the strategy of expansion. Louis IX later referred to as 'Saint Louis', exchanged land in Sommières for land at Aigues-Mortes to build the town. He thus affirmed his authority as far as the Mediterranean, which had remained outside the royal domain. The castle was then associated with royal power until 1791.
In 1220, the royal administration confiscated the first half of the town, the property of Bernard VII of Anduze. The first building operations began: the apron wall of Bermond tower, the chapel, accommodation and the west curtain wall. A second stage of building work started in 1239 – 1241: La Vignasse tower, a faussebraye and Montlaur tower.
In 1248, the king exchanged rights to Sommières with Bermond de Sommières. The royal fortification was about 230 metres long in the 13th and 14th centuries.