Rising above Ostia's medieval borgo (village), this landmark castle is a fine example of Renaissance military architecture. It's named after Pope Giulio II, who had it built at the end of the 15th century to guard the mouth of the Tiber, then one of the main access routes to Rome. It was eventually abandoned after the river's course was changed by heavy flooding in 1557.
Admission is by guided tour only, with visits starting at 9am, 10.30am and noon.