Dungeon Houdan is a medieval fortified tower located in the Yvelines Houdan.
Built in the early twelfth century by Amaury III de Montfort is one of the last vestiges of the old fortifications of the city. It is a massive tower, isolated west of the town. It is currently used as a water reservoir. This is not strictly speaking a dungeon because it is not part of a castle.
Houdan the dungeon consists of a cylindrical tower apparently 16 m diameter and 24 m in height with thick walls of 3.50 m, flanked by four towers of the same height of 4.80 m in diameter.
Read More... http://fr.wikipedia.org/wiki/Donjon_de_Houdan