The Didymoteicho Fortress or Castle, is an ancient and medieval hilltop citadel complex in the town of Didymoteicho, Thrace, Greece. It has been an important landmark since ancient times due to the strong fortification surrounding it. The castle is accompanied by several myths, one of the most famous is that of the Forty Arches, and is where Charles XII, King of Sweden, is said to have been imprisoned by the Turks.