As a result of intense pirate activity in the seventh to ninth centuries AD and the activities of the Knights of St John, the island has a remarkable number of castles and fortresses of which many have survived. The seven principal ones are at Rodos Town, Archangelos, Feraklos, Lindos, Asklipios, Monolithos and Kritinia.
Defence against the Moslems was an ongoing concern of the Knights which spurred them into almost constant building and reinforcing. Twelve fortresses were known to exist under Grand Master E.B. Degli Orsini in 1479 which had to serve 44 settlements but the full total was thought to be around 16.
Each settlement was instructed on evacuation procedures and assigned to the various castles. Settlements at Gennadion, Kalathos and Lardos were assigned to Lindos castle for example. Other castles mentioned in these instructions were located at Apolakia, Lahania, Katavia, Apollona, Salakos, Fanai and Villanova as well as the fortifications at Filerimos.
In addition to these, there are castle remains found at Lardos and Mesanagros. Towers were also built around coastal regions, especially on the west coast as an integral part of the elaborate defenses. Some of these are still in good condition.
Apart from the medieval castles, there are a number of other castle ruins, especially nearby the village of Kalithies: The Erimocastro castle with ruins dating back to Classical and Hellenistic times, Mesovounos with ruins dating back to the Prehistoric and Classical times, and Sarantapichos with the ruins of a necropolis.