Bodrum Castle () is a historical fortification located in southwest Turkey in the port city of Bodrum, built from 1402 onwards, by the Knights of St John (Knights Hospitaller) as the Castle of St. Peter or Petronium. A transnational effort, it has four towers known as the English, French, German, and Italian towers, bearing the names of the nations responsible for their construction. The castle was completed in the late 15th century, only to be taken over by the Islamic Ottoman Empire in 1523. The chapel was converted to a mosque, and a minaret was added. The castle remained under the empire for almost 400 years. After remaining empty following World War I, in the early 1960s, the castle became the home for the award-winning Bodrum Museum of Underwater Archaeology (see below). In 2016 it was inscribed in the Tentative list of World Heritage Sites in Turkey. History Confronted by the now firmly established Ottoman Sultanate, the Knights Hospitaller, whose headquarters were on the island of Rhodes, needed another stronghold on the mainland. Grand Master Philibert de Naillac (1396–1421) identified a suitable site across from the island of Kos, where a castle had already been built by the Order. Its location was the site of a fortification in Doric times (1110 BC) as well as of a small Seljuk castle in the 11th century. The same promontory is also the probable site of the Palace of Mausolos, the famous King of Caria. In his travel diary Travels In Asia Minor, Charles Boileau Elliot describes this palace as the Palace of Mausolus with absolute certainty, and this account was written in the year 1840.The construction of the castle began in 1404 under the supervision of the German knight architect Heinrich Schlegelholt. Construction workers were guaranteed a reservation in heaven by a Papal Decree of 1409. They used squared green volcanic stone, marble columns and reliefs from the nearby Mausoleum of Halicarnassus to fortify the castle. The first walls were completed in 1437. The chapel was among the first completed inner structures (probably 1406). It consists of a vaulted nave and an apse. The chapel was reconstructed in Gothic style by Spanish Knights of Malta in 1519–1520. Their names can be found on two cornerstones of the façade. Fourteen cisterns for collecting rainwater were excavated in the rocks under the castle. This was a monumental achievement of the day and the family