Didymoteichon (“Twin-Wall”) was a city of Thrace, located on a hill above the river Erythros, a tributary of the Hebros. Giannopoulos assumes that Justinian I built the fortress to replace Plotinopolis, situated in a valley somewhere nearby but difficult to defend. By the 9th century Plotinopolis was completely replaced by Didymoteichon; in fact it had probably already disappeared in the 7th century, and the mention of Ploutinoupolis by Constantine VII Porphyrogennetos is anachronistic. The kastron of Didymoteichon was taken by Krum in 813; its first known bishop, Nikephoros, participated in the council of 879. The fortress is mentioned as the place of exile for Bardas Skleros. From the end of the 12th century Didymoteichon appears as one of the most important strongholds in the area, and a sought-after prize for Latins, Bulgarians, rulers of Epiros, and John III Vatatzes, who occupied it around 1243. During the Civil War of 1341-47 Didymoteichon was the headquarters of John VI Kantakouzenos who was crowned there in 1341. Matthew I Kantakouzenos held Didymoteichon as his appanage. In the 14th century Didymoteichon consisted of the citadel and a lower city that was destroyed during the civil war and rebuilt in 1342. The city was temporarily taken by the Turks in 1359 and permanently conquered in 1361. The bishop of Didymoteichon became archbishop in the 12th century and metropolitan in the 13th century. From 1204 to 1206 Didymoteichon was the residence of the patriarch John X Kamateros and a center of resistance to the Latin patriarchate of Constantinople. Some remains of the medieval city survive, including traces of the double city wall with towers and gates, a water reservoir of stone and brick, and the Church of St. Catherine of around1300.
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Oxford Dictionary of Byzantium