Sinope was a major port of Pontos whose double harbor and location at the narrowest point of the Black Sea provided commercial importance and close ties with the Crimea. Its early history is obscure. It appears in written sources in connection with the Black Sea: Justinian II used Sinope to reconnoiter Cherson, and a kommerkiarios of Sinope and the Black Sea is named on a 9th-century seal. Sinope was involved in the revolt of Armeniakon in 793, and in 834 Theophobos was proclaimed basileus of Sinope by "Persian" mercenaries. Sinope lay outside the main Arab invasion routes, though they did attack it in 858. In 1081, the Seljuks captured Sinope along with a sizable imperial treasury established there. Alexios I restored Byzantine rule, and Sinope prospered as a well-defended port; it was the base for Andronikos (I) Komnenos during his activities in the Pontos. The Komnenoi of Trebizond held Sinope from 1204 to 1214, when it fell to the Seljuks; except for a brief Trapezuntine recapture around 1254-65, it remained under Turkish rule. Sinope was a suffragan bishopric of Amaseia. Its main Byzantine monuments are the fortifications and a gymnasium.
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Oxford Dictionary of Byzantium