Château de Dinan, Medieval castle in Dinan, France

Château de Dinan is a composite castle on the town's southern edge, formed in the late 16th century by Duke de Mercœur from three originally separate elements: the ducal keep built in the 1380s by Duke John IV, the Porte du Guichet and the Tour Coëtquen. The keep, over 30 m high, consists of two joined round towers with a square forework at the west junction; its crown carries four-stepped machicolation corbels. A steep timber hood covers the arched opening at the base.

The present keep was built from 1380 under Duke John IV, who chose Dinan to assert his authority over a town that had long backed his rival Charles of Blois; master mason Étienne le Tur completed it by 1393, perhaps as early as 1384. In 1595–98 the Duke of Mercœur joined it to the Porte du Guichet and the Tour Coëtquen, walling up the gate. Military engineer Garangeau later proposed turning the tower into a governor's residence and then a military prison, where hundreds of English sailors were held during the 18th century.

The keep is called the Tour de la duchesse Anne, though it was begun for Duke John IV; the name recalls Anne of Brittany, duchess twice over. The building was conceived as a princely tower-palace, not merely a fortress: its interior layout and carved decoration served the court of the Montfort dukes. From 1908 the town's museum of history and crafts occupied it, and its ethnographic collections from the Rance valley were moved to municipal stores in 2015.

The keep stands at the south end of the old town, next to the Porte du Guichet and the Tour de Coëtquen, within the 2,600 m of ramparts. The visit is a walk through the tower-palace rooms, and the terrace that replaced the slate roof in the 18th century gives a view over the town and the Rance.

The keep's lower machicolation corbels are drawn out very long, resting on many stone courses; the French sources single this out as a deliberate aesthetic effect. The keep was later topped by a terrace instead of its original slate roof, and the chapel apse was pierced for a new entrance when it became a prison.