Coordinates: Helenopolis (Greek: ?) or Drepana (?) or Drepanon () was an ancient Greco-Roman and Byzantine town and bishopric in Bithynia, Asia Minor, on the southern side of the Gulf of Astacus. It has been identified with the modern village of Hersek, in the district of Alt?nova, Yalova Province. It is traditionally considered as the birthplace of Saint Helena.
According to the 6th-century historian Procopius, Helena's son Emperor Constantine the Great renamed the city "Helenopolis" to honor her birthplace; but the name may simply have honored her without marking her birthplace. Constantine also built there a church in honour of the martyr Saint Lucian; it soon grew in importance, and Constantine lived there very often towards the end of his life.
Emperor Justinian built there an aqueduct, baths and other monuments. It does not seem ever to have grown, and it was slightingly called (a pun on its name) Eleinou Polis, "the wretched town".
Nearby, in the late 11th century, Alexios I Komnenos built a castle called Kibatos or Civetot for Anglo-Saxon mercenaries who had opted to flee England after the Norman Conquest and serve the Byzantine Emperor. In 2019, an academic survey identified the remains of Kibatos/Civetot 3.5 meters underwater in Hersek Lagoon. The remains of the castle span approximately 4,200 square meters and were identified based on architectural similarities to contemporary descriptions. In addition to the discovery of the castle - believed to have been abandoned due to earthquakes at an indeterminate time - among other structures, remains of a pier and lighthouse were found, which were visited by Evliya Çelebi and are known to have been used from the Byzantine period right up until the demise of the Ottoman Empire.
Michel Le Quien mentions nine of its bishops. Macrinus, the first, is said to have been at the Council of Nicaea (325), but his name is not given in the authentic lists of the members of the council. About 400, the church of Helenopolis was governed by Palladius of Galatia, the friend and defender of John Chrysostom, and author of the Historia Lausiaca. The last known bishop assisted at the Council of Constantinople (879-880). Helenopolis occurs in the Notitiae Episcopatuum until the twelfth and thirteenth centuries.