The most prominent structure on the harbor in Kos Town is a huge Crusader castle built by the Knights of St. John. The Castle of Neratzia was built between the late 1300s and early 1500s. The castle once occupied its own small island. Access to the castle is via a bridge. Once across the bridge, you come to the admissions booth. A ticket costs €3 (as of October 2012).
The castle is actually two castles in one. The inner castle is the oldest part of the castle, built in the late 1300s to defend the island against Ottoman Sultan Yıldırım Beyazıt. Only three of the original four towers remain from the old castle, as does the wall facing the sea.
Throughout the early 1500s, the castle was reinforced with outer walls, leaving a huge open space between the new and old walls.
The castle was built using materials from ancient ruins in town and from the Asklepeion. If you pay close attention to the walls during your visit, it’s easy to spot some inscriptions in ancient Greek built into them. There are also tablets with the Coat of Arms of the Knights and an Ottoman prayer niche.
Within the courtyards of the castle, there are several pieces of ancient columns and tombstones scattered about. You’ll also find Ottoman cannons and cannonballs.