Cyzicus (sĬz´Ĭkəs), ancient city, NW Turkey, at the neck of the Cyzicus Peninsula. Strategically located, it rivaled Byzantium in commercial importance. Founded (756 BC) by Greek colonists from Miletus, it was a member of the Delian League. In 410 BC, Alcibiades defeated a Spartan fleet off Cyzicus, and in 74 BC the city withstood a siege by Mithradates VI of Pontus. As a reward for its loyalty, it became a free city under Roman rule. In AD 675 the town was pillaged by the Arabs and then used as a base for a siege of Constantinople. There are ruins of a large temple built by Roman Emperor Hadrian and of other public buildings.