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.