Located at the entrance of Kos harbour, on a promontory which in Antiquity was an island; it was joined to the land with a bridge which survives to this day (Foinikon St. Bridge). It consists of two concentric baileys. The inner one has four corner towers, of which the SE has been incorporated into the outer bailey wall. The outer bailey is more massive, with bulky bastions on the four corners, and has battlements with embrasures. A wide moat separates the two baileys and is crossed by a ramp supported on arches.

The castle was constructed with locally quarried stone and the masonry incorporates numerous decorated architectural elements (columns, inscriptions, epistyles, bases etc.), derived from the ruins of the ancient city. Several heraldic slabs are also mounted on the walls.

Over the main gate has been set a Hellenistic frieze with masks and garlands. The roofing of gates (main gate, Carmadino Gate) consists of granite columns placed horizontally side-by-side; they probably came from the Early Christian basilica of the harbour.

We know from travellers? accounts that the inner bailey must have been built before the end of the 14th century, when Ottoman sultan Bayazid I was growing more aggressive. Nevertheless, the oldest surviving part of the castle is the round tower immediately to the left of the ramp connecting the two baileys and bears the arms of grand masters Jean de Lastic (1437-1454) and Jacques de Milly (1454-1461). The outer bailey is later in date; its construction began under grand master Pierre d?Aubusson around 1495, was continued by Emery d?Amboise and completed under Fabrizio del Carretto in 1514.

Between the inner and outer bailey, on the north side, stood a Hospitaller building (storehouse) which was restored during the Italian occupation of the island in the first half of the 20th century and today shelters the exhibit of ancient architectural members of Kos. It is open to visitors and also serves as a temporary display of altars, sculptures and inscriptions.