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.

[4] They used squared green volcanic stone, marble columns and reliefs from the nearby Mausoleum of Halicarnassus to fortify the castle.

This was a monumental achievement of the day and the family who completed the excavation were given the honorific of "Burrows" for their exceptional digging skills.

Each tongue, each headed by a Bailiff, was responsible for the maintenance and defense of a specific portion of the fortress and for manning it with sufficient numbers of knights and soldiers.

The architect applied the latest features in castle design; the passages leading to the gates were full of twists and turns.

Two hundred and forty-nine separate designs still remain, including those of grand masters, castle commandants, countries, and personal coat of arms of knights and religious figures.

Grand Master Fabrizio del Carretto (1513–21) built a round bastion to strengthen the land side of the fortress.

These included twelve slabs of the Amazonomachy (combat between Amazons and Greeks) and a single block of the Centauromachy, a few standing lions, and one running leopard.

When faced with an attack from Sultan Suleiman, Philippe Villiers de L'Isle-Adam, the Grand Master of the Knights Hospitallers, ordered the Castle to be strengthened again.

The terms of surrender included the handing over of the Knights' fortresses in Kos and St Peter's Castle in Bodrum.

In 1846 Lord Canning, HM Ambassador to Constantinople, obtained permission to remove twelve marble reliefs showing a combat between Greeks and Amazons from the castle.

Sir Charles Newton, a member of the staff of the British Museum, conducted excavations and removed a number of stone lions and one leopard in 1856.

[citation needed] In 1962 the Turkish Government decided to turn the castle into a museum for the underwater discoveries of ancient shipwrecks in the Aegean Sea.

This has become the Bodrum Museum of Underwater Archaeology,[6] with a collection of amphoras, ancient glass, bronze, clay, and iron items.

[citation needed] Most of its collection dates from underwater excavations carried out by the Institute of Nautical Archaeology (INA) after 1960.