In the 13th century, the design of the castle () evolved in response to contact with the more sophisticated fortifications of the Byzantine Empire and the Islamic world during the Crusades. These new fortifications were more geometric, with a central high tower called a keep () which could be defended even if the curtain walls of the castle were breached. The donjon of the Château de Vincennes, begun by Philip VI of France was a good example.It was high, and, even though within the moat and walls of the fortress, had its own separate drawbridge to going to higher floor.