The most impressive and imposing building on the island is the Castle of the Knights, which stands on the north-eastern side of the harbour, ideally situated to defend Kos town and control the narrow strait between the island and the mainland of Asia Minor. It was built by the Knights of St. John.
The knights arrived on Kos towards the middle of the 14th century. They immediately realised how important it would be to fortify the town and thus protect the island. We cannot be sure that they began to build on an empty site: they may well have found the remains of the walls of a Byzantine (or even earlier) fortress, which they incorporated into their own structure. However we know that they had no hesitation in using sections from ancient buildings, which became part of their walls.
Today, many ancient stones, pieces of marble, sections of columns and other architectural members from ancient monuments can be seen in the medieval castle. One can only guess at the severity and extent of the damage which the Knights caused to the ancient monuments of the island (destruction which the Turks continued still more systematically at a later date).
The Castle as we see it today consists of two main groups of buildings, an inner (and older) group, and an outer group. The inner enceinte, which is rectangular in shape, was begun in the time of Fantino Guerini,Venetian governor of Kos in 1436 – 1453. Work was continued by his successors, and completed in 1478, under the Grand Master Edoardo di Carmadino. Later it was concluded that an outer ring of defences would also be necessary to protect the inner enceinte against the developments which had occurred in the military arts in the meantime (and, in particular, against the greater firepower of the artillery).
And so the building of the outer enceinte, with stronger walls, began in around 1495, under the command of Grand Master d’Aubusson. Construction continued through the time of d’Ambois and further additions were made in 1514, under Del Carretto.
The fact that the work of building the Castle went on for many decades under a number of Grand Masters explains why there are so many coats-of-arms at various places on the inner and outer walls.
Under the Turkish occupation, which began in 1523, the new owners of the Castle repaired its walls and continued to use it. Indeed, for many years it was forbidden for Christians to enter the walls of the Castle.
In the Middle Ages there was a moat on the city side of the castle (where Phoinikon Avenue is today), filled with sea water. In this way the castle appeared to be standing on a little island.
The main entrance faced the “plane tree of Hippocrates” There were three arched bridges and a narrow passage, together with a drawbridge.
Today, we can see sections of the walls, towers, battlement and staircases, together with a series of galleries, roads, corridors and gates in the inner outer enceintes.
The most impressive view of the Castle of the Knights on Kos is to be had by walking along the western section of the outer wall.
The most imposing of the towers is that of Del Carretto, a semi-circular structure in the south-west corner of the outer enceinte.
Under the bridge which leads to the castle gate, in Phoinikon Avenue, and in many other parts of Kos town we can see remnants of the circular wall which the Knights built between 1391 and 1396 in order to protect the town from attack.