Kos or Cos is a Greek island, part of the Dodecanese island chain in the southeastern Aegean Sea, off the Anatolian coast of Turkey. The principal town of the island and seat of the municipality is Kos town. The island has a 14th-century fortress at the entrance to its harbour, erected in 1315 by the Knights Hospitaller, and another from the Byzantine period in Antimachia. The Ancient Agora. The ancient market place of Kos was considered one of the biggest in the ancient world. In the western side excavations led to the findings of rooms with mosaic floors which showed beastfights, a theme quite popular in Kos.