Inland citadel located approximately in the centre of the island, about 9 km distant from the coast, on a hill 360 metres high. It is assumed that the hill was fortified in Mycenaean times; a section of wall has been traced on the NW slope and remnants of cyclopean masonry on the NE slope. There are also Byzantine and Hospitaller building phases. There are three baileys. The upper one rings the top of the hill, the second encloses a wider area to the east, west and south, while to the north of the top bailey there are remnants of low walls indicating a third bailey.Gates are visible at three points; the first bailey has four towers and the second bailey three. The most remarkable part of the castle is is the square Byzantine gateway flanked by four semicircular arches. Inside the castle, at the NW corner of the first bailey, stands a rectangular building roughly measuring 5 by 6 metres. It is the only edifice in the castle to have survived complete, but with several alterations and no indication of date. Ti has a flat roof and its only opening is an entrance towards the interior of the bailey. A large underground water cistern is located in the centre of the same bailey and another building, dated after 1522 stands on the SE. Hosios Christodoulos, founder of the monastery of St. John on Patmos in the 11th c., is also credited with the foundation of Pyli as well.