The castle of Antimachia is a remarkable medieval fortress built during the Late Middle Ages from the Knights Hospitaller of St. John. Built on a plateau, southeast of the village of Antimachia with a view to the southern coast of Kos, its position allows for panoramic views over the Aegean archipelago. It was successfully used by the Knights as a fortress in 1457 to prevent an Ottoman attack. According to written testaments of the 14th century, the castle also served as a prison for condemned knights.
The castle has a triangular shape with a peak to the east and every side is organised as an independent fortification line, following the shape of the terrain. It was renovated by the Knights Hospitaller in 1494, after the great earthquake of 1493. The walls were reinforced and new battlements of various types were added along the external wall.
The Castle is impressive on the outside, reminiscent of the battles fought and the constant invasions from conquerors, but there is little saved to see on the inside in good condition, apart from ruins of the old settlement of Antimachia, plenty of space, several cisterns and two churches. The first one is a Byzantine church of Agios Nikolaos (Saint Nicolas) from the 16th century and the second is a chapel of Agia Paraskevi, built in the early 18th century. The old settlement was abandoned in 1840.