Château de Saint-Aubin-du-Cormier, Medieval castle ruins in Saint-Aubin-du-Cormier, France.

The Château de Saint-Aubin-du-Cormier is a castle with a rectangular layout surrounded by multiple defensive towers, of which only the foundation walls remain today. The structure reveals the typical design of a medieval fortification with its thick stone construction and tower bases.

Peter I, Duke of Brittany, ordered the castle built in 1225 to overawe the lordships of Vitré and Fougères, guarding the northeastern frontier of the Duchy of Brittany, the Marches. Taken by Charles de Blois in 1342 and handed to Jean IV de Montfort in 1381, it was remodelled in 1430. After the Breton defeat at Saint-Aubin-du-Cormier in 1488, Charles VIII had it dismantled in 1490.

Local tradition holds that the donjon was mined on the king's orders and split in two, with the face turned toward the victorious kingdom of France kept to humiliate the Bretons; in fact only the north face survives. Legends collected by folklorists Paul Sébillot and Adolphe Orain tell that Anne of Brittany, after the 1488 defeat, fled through a tunnel beneath the castle on a horse shod backwards. In October 2018 the site was filmed for the television programme Secrets d'Histoire.

Only the donjon survives, cut down by mining in 1490, so the standing fragment is the north face; the rest of the 86 by 96 metre quadrilateral and its ten towers are visible as ground traces. The ruin stands on a hilltop near the forest of Rennes, with the valley toward Fougères opening below. The masonry is rough and uneven underfoot, and vegetation grows from the joints.

The fortress was the location of a decisive battle between rival Breton lords and French royal forces. After their victory, the French deliberately dismantled the stronghold to prevent any future rebellion from the region.