The prosperous and successful city in the course of time became a part of the Byzantine Empire. In the 6th century CE the empire declined due to devastation of its population by plague, starting about 541. The peninsula was abandoned by the Hellenes only to be gradually repopulated by Slavs. In the 9th century the Byzantines recovered and reoccupied the peninsula, bringing in Hellenes and Armenians from Asia Minor. They were protected in Roman-style fortified towns called kastra. One of these was Erissos, placed over the site of Akanthos. After it became the site of a Bishopric, Erissos was changed to Ierissos by analogy with Hieros, "sacred."