Under Richard the Lionheart's command, Château Gaillard was built with unbelievable dispatch between 1196 and 1198, securing the western border of English territory until Henry IV ordered its destruction in 1603 (it first fell in 1204). Admission to the inner bailey, upper court and dungeon are by ticket, but there is year-round entry to the château grounds where you can explore the outer bailey. The tourist office has details on tours (€4.50, in French with English-speaking guides), held on request.
Don't miss the unbelievable views of the Seine winding off into the distance and the lovely panorama of the village at the foot of the hill.
The castle was long plundered for its stones as building materials, so that by 1573 it had already dissolved into an uninhabited ruin. In 1862, Château Gaillard was designated a monument historique.