The Leonine City (Latin: Civitas Leonina) is the part of the city of Rome which, during the Middle Ages, was enclosed with the Leonine Wall, built by order of Pope Leo IV in the 9th century. 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.