Leonine city

The Leonine City (Latin: Civitas Leonina) is the part of the city of Rome around which the ninth-century Pope Leo IV commissioned the construction of the Leonine Wall. It is on the opposite side of the Tiber from the seven hills of Rome and was not enclosed within the ancient city's Aurelian Walls, built between 271 and 275. The Vatican City is within the enclosed area, but the Leonine City, containing also the Roman rione of Borgo, is much more extensive than the tiny Vatican City.

The Leonine Wall, which defines the Leonine City, was constructed by Pope Leo IV following the sacking by Muslim raiders of Old St. Peter's Basilica in 846. Built from 848 to 852 as the only extension ever made to the walls of Rome, this three-kilometre wall completely encircled the Vatican Hill for the first time in its history. 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. 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 Christened the Castel Sant'Angelo, up the slopes of the Vatican hill encircling the basilica and descending again to the river. The walling was constructed of tuff and tiling, forty feet high, with 44 strong towers at bowshot intervals. The massive round corner tower that still crowns the Vatican hill has its origins in this construction campaign.

...
Wikipedia