The ancient city of Ostia raised in this territory, established during IV BC as a military camp, in the roman imperial era it develops as a port commercial area, strictly linked to the supply of grain to Rome. Due to the insufficiency of the fluvial port, in 42 AD. Claudius started to build a northern artificial port, connected to Tiber by an artificial canal, and provided with a lighthouse; later a second exagonal harbour, whom remains are stored in Princes Sforza Cesarini private property, was built by Trajan between 106 and 113. At the age of its maximum development, the roman city of Ostia reached 75.000 inhabitants, but in III century there was a demographic decline. It has its upswing in IV century as a residential area, whereas commercial and administrative activities moved to Portus. It was episcopal see since III century. At the end of V century the aqueduct stopped working. In 537, during Goths’s assault, the city was defended by the byzantine general Belisario, even though it was by then impoverished, decayed and depopulated.
The access to Rome from Tiber remains, along which pilgrims merchants and delegations coming from the sea went upriver. In IX century it was sacked by Saracens. Pope Gregorio IV enhance the burg to harbor the salines’ workers alongside via Ostiense, then called Gregoriopolis, and the ancient city was definitely abandoned.
At the end of 400 Bishop Giuliano della Rovere, later Pope under the name of Giulio II, built a castle which took his name, accomplished Basilica Sant’Aurea construction, which started shortly
before at the behest of Cardinal Guglielmo d’Estouteville being located on the burial site of St. Monica nearby St.Aurea burial, and rebuild the city walls. Vasari said that Baccio Pontelli was the architect in charge of this renovation work.
The building maintains its defensive function since 1557, when a flood deviate the river course. The castle lost its commercial function and fall into ruin: it was used as a barn and storage by peasants and shepherds, while the tower was used as a prison. In 800 prisoners were employed in the excavations of the roman city.
Fortress and burg were renovated on several occasions during 900.