Château de Montaillou, Medieval castle in Montaillou, France.

Château de Montaillou is a castle ruin perched on a hilltop in the French Pyrenees with three surviving walls of the main tower and a wide stone platform. The remains reveal how a medieval frontier stronghold was organized across multiple defensive levels.

Originally built to guard the frontier between the County of Foix and neighboring regions, the fortress fell into decline after King Louis XIII ordered its demolition in 1638. This act marked the end of its strategic importance.

In 1309 the Inquisition arrested Montaillou's inhabitants on suspicion of heresy and gathered them in the castle before transferring them to Carcassonne for questioning. The village later became known through Emmanuel Le Roy Ladurie's study Montaillou, village occitan.

The ruins stand at the top of Montaillou, above the village on the slope of Mount Allion, and are reached on foot; there is no staffed reception or visitor facility. The keep walls and the base of the curtain wall sit on uneven ground, so watch your footing near the broken masonry. The site is a monument historique inscrit, listed on 21 December 1984.

In 1309 the Inquisition arrested Montaillou's inhabitants on suspicion of heresy and gathered them in the castle before transferring them to Carcassonne for questioning. A dungeon in the lower part of one tower is documented. The site was listed as a monument historique on 21 December 1984.