The remains of the fortress can be visited
Donjon de Houdan

The Donjon de Houdan is a medieval defensive tower located in the French region of Île-de-France, specifically in the commune of Houdan in the Yvelines department. It is the sole remnant of a former castle complex.
History+
Around the years 1120 to 1137, the donjon was constructed by Amaury III of Montfort. It served as the central keep of a larger castle complex, of which only this tower remains today. In 1840, the structure was classified as an official historical monument. A transformation occurred in 1880 when a 200-kiloliter cistern was installed inside, turning the donjon into a water tower. In 1903, the town of Houdan acquired the structure from its last private owner, Dr. Aulet.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Hervé Rebours
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 48.7892, 1.5990