The old St. Peter's Basilica was constructed across the Tiber in the fourth century. The area around the "Vatican Hill" had been a burial site, but had not been densely urbanized. The Castel Sant'Angelo was, originally, a Mausoleum.

Over the course of the conflicts between the Lombards and the Byzantines, the City of Rome found it necessary to expand the Aurelian Walls to encompass what came to be known as the "Leonine City" (after Pope Leo IV, under whose reign the new walls were constructed). The walls were completed in 852, and we can pinpoint that date as when the City of Rome came to encompass the Vatican Hill, although there must have been some light urbanization prior to that point as well.