Founded in the 11th century, with evidence of its existence from 1021, to control the access roads leading to Carcassonne, the Château d'Aguilar is one of the 'Five Sons of Carcassonne'.
This castrum, composed of a 12th century keep surrounded by a high polygonal wall flanked by semi-circular towers, belonged to the seigneurs of Termes in the 13th century, before being besieged in 1210 by Simon de Montfort.
For 30 years, the castle had no military function and was returned to the Crown of France in 1241.
After becoming a royal fortress in 1257, the castle underwent important modifications at the end of the 13th century in order to defend the Languedoc territory against Aragon. Despite these developments, it bore the full brunt of the fighting between France and Spain in the 14th century and was finally abandoned in 1569, following the Treaty of the Pyrenees, which saw it lose its strategic interest.
Particularly degraded, the Château d'Aguilar, as it can be seen today, comprises 12th century ruins from the era of the seigneurs of Termes, such as the pentagonal wall, and the royal era (the square tower, keep, and second hexagonal wall).