Castle of Rieux, Medieval castle ruin in Rieux, France.

The castle of Rieux is a fortified ruin perched on rocky terrain overlooking the Vilaine River, with several towers and wall sections still standing. The site includes walking paths and interpretative panels that help visitors understand the layout of the former stronghold.

A first castle was built in the 9th century by the lords of Rieux on the site of an ancient oppidum controlling traffic on the Vilaine. French troops destroyed it in 1488 at the end of the Mad War; two years later Anne of Brittany granted Jean IV de Rieux 100,000 écus to rebuild it. Richelieu ordered its demolition in 1629, but only some walls and outer towers were pulled down. The keep collapsed in 1799 after an earthquake.

The castle was the seat of the Rieux family, one of Brittany's leading houses, whose lands stretched through parishes such as Allaire, Bégon, Glénac and Fégréac. Jean IV de Rieux (1447–1518) commanded the Breton army at the Battle of Saint-Aubin-du-Cormier in 1488. A ferry replaced the wooden river crossing after the bridge collapsed in 1543, and tolls were paid to the lords of Rieux.

The site is accessible via walking trails marked throughout the area, and information panels guide you through the ruins and explain what you are seeing. Visit on clear days when views of the river valley are best and the rocky terrain is easiest to navigate.

The entrance gate, or poterne, is the only part of the castle still standing; in the 19th century it was framed by an English garden laid out among the ruins. The site has been a classified natural site since 12 October 1971, and an interpretive trail installed by the commune in the 20th century leads through the remains.