At the entrance of the port of Kos it is situated the so-called «Kastro tis Neratzias» (Neratzia Castle). The place in ancient times was a small islet, connected with the island of Kos by a bridge, the one seen today at Phoinikon Street.

In the 14th century A.D., in the period of sultan Bayazit I, when the raids of the Ottomans on the island had already been dangerous enough, the Knights of the Order of St. John who dominated the island, decided to construct the castle in order to assure the effective defense of the town and the island. Thus, according to travelers’ description, the first precinct had been built in late 14th century. Later on, in late 15th – early 16th centuries, a second precinct was added to surround the first one, with a wide and deep moat between the two. Contact of the two constructions was possible through a drawbridge. The inner precinct has four defensive towers; the southeastern of them had been incorporated to the outer wall, which has huge bastions at the corners, battlements and gun ports.

The material used in the construction is local stone and several architectural parts of ancient buildings. A big frieze of the Hellenistic period has been inlaid at the central gate, while the basalt columns placed obliquely on the ceiling of the gates possibly come from the Early Christian basilica at the port. You may also see several medieval blazons of the knights who dominated the island.