Château de Beynes is a ruined medieval fortress in the Yvelines département, originally erected in 1073 as a strategic defensive stronghold for the French royal domain. Built in an unusual low position in the Mauldre River valley, the castle was fortified by the Montfort family between 1087 and 1092 and successfully defended against English forces. After losing its military significance in the 15th century, it was converted into a residential palace by Robert d'Estouteville and later became a favored estate of Diane de Poitiers under Henry II. By the 18th century it had fallen into ruin as locals quarried its stones. The municipality acquired the site in 1967, initiating archaeological excavations and conservation work that led to its designation as a Monument historique, preserving its importance as a rare example of medieval French castle evolution.
