Largoet Castle in Brittany by EuroTourData
Largoet Castle is the restored ruins of the lowland castle, built around 1260 for the Lord of Elven Derrien.

Image info
© Creative Commons: Elven - forteresse de Largoët 10, author Fab5669. License: CC BY-SA 4.0. Cropped from original.
Description of Largoet Castle
On the territory of the castle there is an octagonal keep with a height of 52 meters and a wall thickness of between 6 and 9 meters. Also in the castle is a four-story butter-churn tower, the so-called Round Tower. In 1474, the famous prisoner was imprisoned in the castle: the Earl of Richmond, the future King of England Henry VII.
Source
Articles Patrimoine Culturel of Elven Commune website on French and Forteresse de Largoët of the official website on French.