Kyzikos (modern Belkıs/Balıkesir) was a city on the southern coast of the Sea of Marmara, at the head of routes leading into Asia Minor. Diocletian made Kyzikos metropolis of the province of Hellespont, headquarters of a legion, and site of an imperial mint. The usurper Prokopios took it in 365; an earthquake destroyed half the city in 539. The Arabs occupied Kyzikos from 671 to 678 during their attack on Constantinople. To repair the devastation, Justinian II installed Cypriot refugees there in 688 and named the settlement Nea loustinianoupolis. In 1078, Kyzikos was base for the attack of Nikephoros Bryennios on Constantinople, and in 1090 and 1113 it briefly fell to the Turks. After ravaging the area in 1204, the Latins rebuilt Kyzikos in 1206; John III Vatatzes recaptured it around 1225. Kyzikos was headquarters of the Catalan Grand Company in 1303-04. Orhan captured it soon after 1335. Kyzikos was the metropolitan bishopric of Hellespont. Its region contained numerous monasteries, notably Megas Agros, the home of Theophanes the Confessor.
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Oxford Dictionary of Byzantium