The Didymoteicho Fortress or Castle (), is an ancient and medieval hilltop citadel complex in the town of Didymoteicho, Eastern Macedonia and 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. Location Located on the hilltop, strategically placed, due in-part to its strategic location overlooking both the town of Didymoteicho, the Erythropotamos River and the road to Constantinople, it is one of the most important Byzantine-era castles in Thrace and the Balkans. History According to the 6th-century historian Procopius, the reconstruction of the walls of Didymoteicho took place during the reign of Justinian I. The castle experienced several long sieges, and as a result The walls were subsequently strengthened during the reign of Constantine V in 751. In 1206, the fortress (known as Demotika) was besieged by Bulgarian Tsar Kaloyan, who took advantage of the disintegration of the Byzantine Empire. The fortress was later occupied by the Latin Empire. In 1303, with the re-establishment of Byzantine rule, master builder Constantine Tarchaneiotes strengthened the fortifications. This is supported by Topography of building materials and methods that suggest the occupation by Byzantine forces continued into the later period of the empire. In October 1352, during the Byzantine civil war of 1352–1357, John V Palaiologos, with military support from Serbia commanded by Gradislav Borilović, engaged John VI Kantakouzenos (with backing from Orhan I, the Ottoman bey). The two armies met in an open field close to the fortress walls. The more numerous Ottomans defeated the Serbs, and Kantakouzenos retained the power, while Palaiologos fled to Venetian-controlled Tenedos. According to Kantakouzenos about 7,000 Serbs fell at the battle (deemed exaggerated), while Nikephoros Gregoras (1295–1360) gave the number as 4,000. The battle was the first major engagement of the Ottomans on European soil, and made Stefan Dušan realize the major growing threat of the Ottomans to Eastern Europe. By 1361, the fortress had fallen to the Ottomans. In a cave next to the 19th-century Cathedral of Saint Athanasios Charles XII, King of Sweden, is said to have been kept under house arrest by the Ottomans. During the Ottoman rule, the structures were not adequately preserved, and by 1848 the fortress was