The medieval castle stands on the SE side of the acropolis of ancient Ialysos, 8 km SW of Rhodes. It is roughly quadrilateral, with towers on three corners. The SE and NW towers were originally polygonal, probably Byzantine in date, while the one NE tower is square in plan. Most of the masonry is probably of the Hospitaller period. The acropolis was a fortified settlement in Byzantine times and the name Phileremos (associated with monasticism) occurs in the written sources from the 13th century. There are ancient, Byzantine, Hospitaller (1309-1522) and Ottoman (16th-19th c.) construction phases. The defensive enclosure was irregular; it measures 700 metres in length and covers an area of 2,000 square metres, with an additional enclosure to the north, with two round towers to the west and east. Within the enclosure survive three churches- Our Lady of Phileremos (15th c.), St. George Chostos (14th c.) and a small 10th c. chapel. Phileremos was besieged by the Genoese in 1248 and by the Knights Hospitaller in 1306. On 11 November of that year he castle was taken through the treachery of a Greek and the garrison, consisting of Turkish mercenaries employed by the Byzantine emperor, were slaughtered. In September 1404, the priory of Our Lady of Phileremos was granted to Knight Fr. Johannes Tensar. The surviving Hospitaller constructions date from 1480 and 1481, under grand master Pierre d?Aubusson (1476-1503). In 1480, the Knights strengthened the defences under the threat of imminent Turkish attack. During the Italian occupation of Rhodes (1912-1943) considerable landscaping and restoration took place (1926 and 1936).