Built in the early 15th century by the Knights Hospitaller (Knights of St. John), the Petronion, better known as Bodrum Castle or the Castle of St. Peter, rises between Bodrum's twin harbors like an illustration from a fairy tale. With German knight-architect Heinrich Schlegelholt at the helm, the knightly builders plundered the Mausoleum at Halicarnassus for green volcanic stone, marble columns, and reliefs to create this showpiece of late-medieval architecture, whose walls are studded with 249 coats of arms, including the crests of the Plantagenets and d'Aubussons. The castle's towers and gardens are visible from many parts of town, and the name "Bodrum" itself likely derives from the word Petronion. Some of the castle's towers are named after the homelands of the knights who built them: France, Germany, Italy, and England (the English Tower, embellished with a relief of a lion, is known as the Lion Tower, and contains a replica of a medieval hall).
The castle now houses the fascinating Museum of Underwater Archaeology, where displays include the world's oldest excavated shipwreck (Uluburun), the tomb of the so-called "Carian Princess," and the sunken cargoes of many ancient and medieval ships that sank off the treacherous Aegean coast.