The towers and crenellated walls of the imposing ramparts of the Rumeli Hisari (1452) can be seen on the European shore of the Bosphorus, resembling a long stone snake lying on the side of a hill. The castle was built in just three months by Mehmet II's workers. Its powerful cannons enabled the sultan to control shipping in the straits. After the fall of Constantinople, the castle was occasionally used as a prison. It now hosts the Istanbul International Festival.