The fortress incorporated the oldest circular tower, built in 1423-24 by Pope Martin V Colonna, which became the Castle of Julius II of Rome.
The rocca housed the papal customs office, which regulated the payment of tariffs for goods arriving in Rome by sea. Upon ascending to the papal throne, Julius II (1503-1513) ordered important transformations: a true papal apartment was built on the west side of the courtyard, remodeling some Borgian-era rooms. The building’s three floors were connected by a monumental staircase, frescoed, according to recent studies, by Baldassare Peruzzi with some collaborators including the Lombard Cesare da Sesto.
At the end of the conflict between France and Spain, the fortress of Ostia underwent a famous siege by the Spaniards in 1556, causing significant damage to the structure. The following year, in 1557, following a spectacular flood, the Tiber changed its course, moving away to follow a new path that remains today. This caused the transfer of the papal customs first to Tor Boacciana and then to Tor S. Michele.
In the 18th century, the castle was used as a hay barn and then in the following century became a prison for forced laborers employed in the excavations of ancient Ostia. After various restorations carried out during the 20th century, a new museum setup was created in 2003 in some rooms of the papal apartments and the keep, displaying the most significant part of the collection of late medieval and Renaissance ceramics, recovered from excavations conducted inside the castle and the borough during the previous century.
