Leonine City

[1] This area was located on the opposite side of the Tiber from the seven hills of Rome, and had not been enclosed within the ancient city's Aurelian Walls, built between 271 and 275.

After Christianity had risen to prominence and the Western Roman Empire had collapsed, the area had to be defended through the construction of a new wall, since it housed St. Peter's Basilica.

[3] An abortive start had been made by Leo III, but disturbances in the city had suspended work, and the Romans dismantled the sections that had been begun and used them in private constructions.

[4] Pope Leo IV used his estate workers, inhabitants from the surrounding countryside, Saracens captured after the sea battle of Ostia in 849 and funding from an imperial Frankish donation, to construct the wall, which ran in an enclosing U-shape from the riverbank at Hadrian's Mausoleum, soon to be known as the Castel Sant'Angelo, up the slopes of the Vatican Hill encircling the basilica and descending again to the river.

[7] In 1083, after refusing to crown Henry IV as the next Holy Roman Emperor, Pope Gregory VII found himself under siege within the Leonine City.