This is a 13th-century block of flats in the heart of the castle, which defense terminology calls "acropyrgos" or "donjon". It was used as the duke of Naxos’s final shelter. Its height is initially considered to be very tall, allowing for the observation of the surrounding sea area, but during the Ottoman rule it fell into decay and was partially demolished, while in the 20th century it functioned as an aqueduct. Up to now, spools (second-hand building materials) have been distinguished from the ancient columns to have been used during its construction.