The name given to that portion of Rome comprising the Vatican basilica and palace with the surrounding suburb, which Leo IV. (847-55) enclosed with a wall. In the time of Leo III. (795-816) the frequent descents made by the Saracens on the coast of Italy inspired that pontiff with the design of securing against their attacks the churches and religious establishments outside the walls of Rome. Shortly before his accession to the pontificate the Saracens ascended the Tiber and plundered the basilica of St. Peter on the Vatican and that of St. Paul without the wall. Leo III. having informed the emperor Lothaire of this, the latter encouraged him to enclose these churches within the circuit of the walls, and sent large contributions in money from himself and his brothers for that purpose. Leo consulted the Romans, called in the peasants from the Cam-pagna, and labored for four years (848-'52) in constructing the fortifications round the Vatican and the adjoining suburb, which he connected with the city. A tax was also levied on the entire duchy of Rome to defray the cost of the work.

This part of Rome was hence called the Leonine City. It was still further fortified and embellished by Nicholas V. (1447-55). It was the district inhabited by the Anglo-Saxon pilgrims during the early ages; hence the name of Sassia applied to it afterward. It also contains the Giraud palace, built by Bramante, which was the residence of the English ambassador before the reformation.