Many other structures date back to Greek occupation in ancient times. Ancient Greeks inhabited Giresun, then called Kerasous, from the 5th century BCE. During this time, they must also have used Giresun Island. The poet Apollonius of Rhodes mentioned this island in his best-known epic, the "Argonautica." Altars on the island date to the Classical or Hellenistic period. Its use as a religious center continued after the rise of Christianity in the region. During Byzantine times, likely in the 400s or 500s, a monastic complex was built on the island, dedicated to either St Phocas of Sinope or Mary. It functioned both as a religious center and as a fortress.