The castle is built on the original site of antique Halicarnassos by the Knights of Rhodes in the 15th century who dedicated it to St Peter. Because the knights were from different nationalities, the towers of the castle were named French, English, Italian, and German. The castle covers an area of about 30,000 square metres. Stones and relief panels from the Mausoleum of King Mausolos were used in its construction.
Today the castle houses the Bodrum Underwater Archaeological Museum. This is a special museum displaying finds from excavations which began in 1960, a collection of the eastern Mediterranean amphora and remains of the oldest ship-wreck in the world.