After the sack of Constantinople by the Crusaders in 1204 new rulers settled in the Peloponnese, the Ionian and the Aegean islands and tended to found their defensive network upon the pre-existing and added new wherever they thought it was necessary, complying with contemporary military techniques and weapons. The Venetians established mainly commercial bases allowing them access to Venice and control of commercial routes. The Knights Hospitallers on the other hand after being expelled from the coast of Asia Minor constructed purely military defensive structures in order to protect themselves, the island inhabitants and the commercial routes.
New construction was more numerous in the islands of the Dodecanese where the Knights of the Hospitallers of Saint John (ex-Jerusalem) had to defend a specific area as close to the Holy Land as possible. On the ten islands of the Dodecanese which made up the empire of the Knights, 56 castles, including fortified monasteries are known. 21 of them are located in Rhodes and 35 in the other nine islands. Only two of those can securely be dated to between the late 7th and 9th centuries. According to historical sources 6 more castles were built earlier than the period of the Hospitallers. These include: on Rhodes: Lindos, Faraklo and Filerimos, Pili Castle on Kos, and on Leros, Pandeli and Lepedis Castles. Most of these castles were purely defensive and not designed to withstand a long siege or to overcome the invaders. Castles served as refuge for the people when the whole island or the castle itself was under attack. An exception to this was in 1458 on the island of Symi when the castle repelled an invading force of 7,000 Ottoman Turks. The Turks finally captured Rhodes in 1522 thus ending the Hospitallers ascendancy.