Château de Mirabat, Medieval castle on a ridge in Seix, France.

Château de Mirabat is a ruined castle on a rocky outcrop at 1,242 m, at the tripoint of Seix, Oust and Ustou in the Ariège. Only the remains of its curtain wall survive, without corner turrets, built of white marble from quarries opened on the spot. The jagged masonry stands on the ridge, its base partly buried by snow in winter and overgrown with dark vegetation.

Mirabat is the oldest castle in the Ariège and among the oldest in the Pyrenees, probably preceded by a fire tower said to have been built on Charlemagne's orders. It was likely raised to watch for Moorish incursions through the Salau pass, and in the 12th century it became a garrison of the Viscount of Couserans. In the 12th and 13th centuries its keep dominated the Haut-Salat under the Roger de Comminges; it was abandoned in the 14th century, probably because of its difficult access.

The name Mirabat comes from Occitan mira abat, "look down", after the castle's position over the Salat valley. In 1667 Louis de Froidour, Louis XIV's forest commissioner, was told the castle served to light fires at night or make smoke by day as a warning signal. The hamlet of La Taula probably takes its name from an old toll post at the crossing of transborder paths.

The castle is reached only on foot, by a long hike; the ruins stand at 1,242 m near the tripoint of Oust, Seix and Ustou, and the usual approach is from Seix. Sturdy footwear and several hours are needed, and the difficult access is probably why the site was abandoned from the 14th century.

An ancient Roman ceramic vessel was discovered in the water tank at the site, suggesting the location was occupied and valued centuries before the fortress was built. This find hints that the ridge held military or administrative importance during classical times.