In the middle of the XV century on the site of the former Seljuk fortress loannites Knights built a castle of St. Peter of green granite. At the double walls of the fortress. Towers are called as belonging to the countries belonging to the Order of the Knights of St. John: English, French, German and Italian. In addition, there is another tower - Estuary, or the Port Tower, which today serves as the main entrance to the castle. More one-sided - the so-called "North entrance with a moat," overlooking the main square of Bodrum and serves as the entrance to the castle from the shore.

In its heyday, the castle was inhabited by perhaps 50 knights from seven different European countries and three times as large number of rank and file soldiers. Their main task was the defense of the castle and its surroundings.

In 1453 the castle was the only Christian stronghold in Anatolia. At this time, it was reconstructed, strengthened and built 14 tanks to store water during the siege. However, in 1522 the castle surrendered and was soon abandoned. In the XIX century the castle was converted into a prison and a chapel in the castle rebuilt into a mosque.

Since 1960 here the Museum of Underwater Archaeology, which is stored underwater finds: amphorae, raised from the seabed, coins and weapons. On the open alle ancient sarcophagi are exhibited, including a sarcophagus with a skeleton of Princess Ada, sister of the legendary Mausolus.