The castle of Kos, better known as the castle of Nerantzia (castle of the sour orange tree), is the most important part of the fortifications constructed on the island by the Knights Templar at the end of the 14th century A.D. It ows its name to the many sour orange trees that were planted in Kos during the late years of the 14th century and the early years of the 15th century, when the whole city came to be called by this name.The castle consists of two fortified curtain walls. The inner curtain wall is the oldest. It is rectangular shaped and on its corners boasts 4 high towers. A wide moat (ditch) seperates the two curtain walls which are connected only by a single inclined road on the eastern side of the castle.
The castle of Nerantzia along with the castle of Saint Peter on the opposite coast of Ancient Alicarnasus (today's Bodrum Castle) controlled the sea way towards the holly lands during the period of the crusades.
In the construction of the castle various architectural elements from the ancient city of Kos and the Asclepieion were used. Many of these can still be seen today embedded in the walls of the castle.
During the 19th century the castle was used as the barracks of the Turkish garrisson and as the home of the Turkish commander of the island.
On March 17th 1816 a gun poweder room exploded by accident and destroyed a large part of castle. The castle's current state is a result of the restoration efforts of the Italian Arcaheological Department, which, during the Italian occupation period, removed the additions that had been created during the Ottoman era and tried to restore the castle to its original state and form.
During the German occupation, the castle hosted the German garrison and was used as a prison for the emprisonment of Greek patriots.