The Knights Hospitaller did not build as a single people. They built as a federation. Each langue, or tongue, of the Order, raised its own tower in its own style and was responsible for defending its own stretch of wall. The English, French, German, and Italian towers still rise above Bodrum harbor, each bearing the heraldry of its makers. Above the door of the English Tower, called the Lion Tower for the antique relief carved into its western face, sits the coat of arms of King Henry IV. The Spanish Knights of Malta reconstructed the chapel in Gothic style between 1519 and 1520, signing their work on two cornerstones of the facade. Two hundred and forty-nine separate carved coats of arms still survive on the walls. The whole castle reads like a stone diplomatic register of Latin Christendom in the late Middle Ages, every nation on the wall, every nation on the gate.

Long before the Knights arrived, this rocky finger of land mattered. A Doric fortification stood here in 1110 BC. A Seljuk castle was built on the same rock in the 11th century. And somewhere underneath, almost certainly, lay the Palace of Mausolos, the satrap-king of Caria whose fourth-century BC tomb had given its name to every grand burial monument since. The Mausoleum had been one of the Seven Wonders. Earthquakes brought it down across the medieval centuries, and by 1404 it was a vast quarry of green volcanic stone, marble columns, and battle reliefs waiting to be reused. Between 1505 and 1507, the few sculptures that had not yet been smashed and burned for lime were tucked into the castle for decoration. Twelve slabs of an Amazonomachy frieze, a single block of a Centauromachy, standing lions, and a running leopard ended up worked into the masonry. Today most of those pieces sit in the British Museum, removed in 1846 and 1856 by Lord Canning and Sir Charles Newton.

For more than a century, St. Peter's Castle was the second most important holding of the Order, after Rhodes itself, and the only refuge for Christians along the coast of Asia Minor. It stood through the shock of Constantinople's fall in 1453 and held against Mehmed II's attack in 1480. In 1482, a strange guest arrived: Prince Cem Sultan, son of Mehmed II, fleeing his own brother after a failed bid for the Ottoman throne. The Knights took him in. Forty years later, the math caught up. When Suleiman the Magnificent moved on Rhodes in 1522 with two hundred thousand soldiers, the headquarters fell in December. The terms of surrender were pitiless in their efficiency. The Knights would leave Rhodes, Kos, and St. Peter's Castle without a fight. Bodrum was handed over without ever being besieged.

Under the Ottomans, the chapel became a mosque called the Suleymaniye Camii, named for the sultan who had taken it. A minaret rose where Knights had once celebrated mass. The traveler Evliya Chelebi recorded the place in 1671. The castle would serve the empire for nearly four hundred years, with stints as a military base, a hamam, and finally a prison after 1895. Then on 26 May 1915, in the middle of the First World War, a French warship offshore opened fire. The minaret came down. Several towers were damaged. The Italians garrisoned the empty ruin briefly after the war and withdrew in 1921 as Mustafa Kemal consolidated the new Turkish republic. For four decades the castle simply stood, unused, looking out over Bodrum harbor. The minaret was reconstructed in its original form in 1997.

In 1962, the Turkish government turned the castle into something it had never been: a museum dedicated to what lay below the waves. The Bodrum Museum of Underwater Archaeology grew out of pioneering excavations by the Institute of Nautical Archaeology, beginning with the Cape Gelidonya wreck, excavated starting in 1960, the first scientifically complete underwater dig in Turkey. The collection now spans four millennia of Mediterranean shipwreck. The Uluburun ship, lost off the southern Turkish coast in the 14th century BC, gave up ten tons of Cypriot copper ingots, a ton of pure tin, 150 glass ingots, Mycenaean pottery, and a gold scarab carrying the seal of Queen Nefertiti. A Carian princess who died between 360 and 325 BC has her own room. The chapel that became a mosque now holds Bronze Age amphoras. From the Italian Tower, you look out over the bay where many of these ships sank. The stones the Knights stole from a tomb keep the trade goods of vanished sailors.