Herakleia (ancient Perinthos, modern Marmara Eregli) was a city on the north shore of the Sea of Marmara, at the junction of the Via Egnatia and the main Balkan road to Nassius.
Renamed Herakleia by Diocletian (who was Herculius in official terminology), it continued to be called Perinthos by antiquarians up to the mid-15th century. According to Procopius, it had been the most important city in the province of Europa, but was replaced by Constantinople, which was originally its suffragan. A bishopric in 325, Herakleia appeared as a metropolis in notitiae: the number of its suffragans increased, but Constantinople became independent of Herakleia in 330 or 381.
Herakleia was attacked by the Goths after the battle of Adrianople in 378, then by Attila, by the Avars, and the Bulgars. The city is mentioned by many later authors mostly as a geographical site or an ecclesiastical center. The citizens of Herakleia supported Thomas the Slav against Michael II. In the Partitio Romaniae, "Yraclee" was assigned to the Venetians. The city played an important role during the civil wars of the 14th century. In 1382, together some other Thracian towns, Herakleia was given over to Andronikos IV. The remains of an aqueduct and at least one church—perhaps that of St. Glykeria, damaged by the Avars in 591 and rebuilt by Maurice—have been preserved.
Oxford Byzantine Dictionary