The castle of Kos, known as the castle of Nerantzia, was built by the Knights of St. John in the late 14th century and is the most important part of the Knights' Fortification. At the end of the 14th and the beginning of the 15th century, Kos was densely planted with lemon trees and bitter orange trees (nerantzi), and it was named Nerantzia and its Castle of Nerantzia.
The castle consists of two fortified enclosures, the interior is earlier. It has a rectangular shape and at the corners it is framed by 4 tall towers. A wide trench separates the two enclosures and only on the eastern side are joined by a sloping downhill road.
Many architectural members from the ancient city of Kos and Asklepieion have been used to build the castle, which are still visible today, embedded in various parts of the castle.
Its current form is due to the Italian Archaeological Service, which during the Italian occupation demolished the Turkish additions and proceeded to extensive restoration of the castle in its original form.
During the German occupation of the castle, the guard was armed and served as a prison. It was proclaimed historically preserved in 1948.