Kos belongs to the Dodecanese group of islands, in the Aegean Sea. The island boasts long sandy beaches with large hotels and secluded villages. The town has a 14th century fortress at the entrance to its harbor, erected in 1315 by The Knights of Saint John of Rhodes. The ancient physician Hippocrates is thought to have been born on Kos, and in the center of the town is the Plane Tree of Hippocrates.