The ruins of Palaeokastro (Greek for "Old Castle") lie approximately 1 km northwest of the harbor bay on a 252 m high hill. A Doric acropolis from the Bronze Age already stood here. In the 4th century BC, Rhodes built a fortification over this acropolis, of which parts of the outer wall ring with the remains of square towers survive, as well as the foundations of another tower, the only clearly recognizable remnant of an inner acropolis. Directly east of the tower is a burial chamber, interpreted as part of a small sanctuary from the 4th century BC. The ancient stonework is recognizable by its carefully hewn ashlar blocks. Within sight of Palaeokastro were the Rhodesian fortifications at the harbor (later covered by the Knights Hospitaller castle) and on the neighboring islands of Ro and Strongyli. It is assumed that the garrisons communicated by means of signals. Numerous cisterns were constructed throughout the site to supply water; the largest, with a staircase hewn from the rock and the remains of a vault above it, is located in the northwest corner of the complex. After the end of Rhodian rule, the island and fortress passed to the Ptolemies and from them to the Romans. Part of the walls were rebuilt during the Byzantine period using ancient materials. The Knights Hospitaller conquered the island in 1307, but apparently, like later owners of Kastellorizo (Catalans, Naples/Aragon, Spain, Ottomans), they left few discernible traces in Palaeokastro. The small Greek Orthodox chapels and house ruins on the eastern side of the site date from the 19th century, as does the large mosaic bearing the year 1858, located between the two chapels "Panagia tou Kastro" and "Agios Stefanos." The remains of an anti-aircraft position from the Second World War, with concrete gun emplacements and brick containers for ready ammunition, are reminders of the Italian occupation of the island (from 1921 onwards). Palaeokastro was used for military purposes until modern times. Consequently, it was shelled and bombed several times in both the First and Second World Wars, resulting in considerable damage to the archaeological remains. Major stabilization and restoration work was carried out in 2013.