The Chateau de Montaillou is a ruined castle in the French village of Montaillou, in the Ariege departement. The village of Montaillou, standing on the slope of Mount Allion, was made famous in Emmanuel Le Roy Ladurie's history, Montaillou, village occitan. At the top of the village is all that remains of the rectangular castle: a ruined tower and evidence of walls and earthworks.
The castle was built by the lords of Alion around the end of the 12th century. Occupying a platform roughly 100m long by 30 to 40 m wide, all that now remains are three walls of the ruined keep. Access was controlled by a dry moat cut into bare rock. The courtyard was surrounded by a curtain wall, the base of which is partially conserved. Further dry moats provided defence to the north and east, while to the south the steep slope of the site was sufficient. The plan of the castle was simple: a wall linked to a tower followed the contours of the hill.
In 1226 Bernard d'Alion paid nominal homage to the King of France, but his sympathies still lay with the Cathars. He married Esclarmonde, daughter of Roger IV, Count of Foix in 1236. The witnesses were Cathar parfaits or at least believers. In 1258, Bernard was condemend by the inquisition as a Cathar heretic and burned alive in Perpignan.
The castle was taken by his father-in-law and became a frontier fortress, between the County of Foix, the French King's lands and Aragon. The first written mention of the castle is in 1272, in a list of the Count of Foix's fortresses. At the end of the 13th century, the Count doubled the thickness of the walls. The castle survived the Albigensian Crusade but fell into disrepair later.