Donjon de Houdan, Medieval keep in Houdan, France

Donjon de Houdan is a cylindrical stone tower that rises about 25 meters high and has small corner structures at each cardinal direction. The walls are roughly three meters thick, creating a solid structure that has survived to the present day.

Built between 1120 and 1137 by Amaury III of Montfort, the tower served as a defensive fortification. Throughout its entire military service, it was never successfully attacked or taken by an enemy.

The keep gave its name to the bourg castral that grew around it, and its four corner turrets are a detail visitors can still trace. Since 2016 the former concrete water tank inside serves as a small cinema showing films about the tower.

The keep stands about 200 metres west of the Saint-Jacques-le-Majeur-et-Saint-Christophe church, isolated from the town centre. Its interior floors and roof are gone, and the original entrance was set 6 metres above ground, so access is through the restoration layout opened in 2016. The concrete water tank installed in 1952 now serves as a film projection room.

The keep's military life ended in 1880, when a 200,000-litre metal cistern was fitted inside and it became Houdan's water tower. In 1952 a reinforced-concrete tank replaced it, mounted on a frame kept clear of the medieval walls. That tank now serves as a film projection room.