The castle of Dourdan was built about 1222 for Philippe Auguste. Kings and large lords were the successive owners. At the end of the XVIIe century, it was given to the duke of Orleans which transformed it into prison. This function remained until 1852. This fortress, classified Historic building in 1964, is a remarkable example of military architecture which preserved its courtines, its turns its ditches and especially a circular keep isolated from the castle itself. All the defensive system is thus still visible.