The name "Vatican" was already in use in the time of the Roman Republic for the "Ager Vaticanus", a marshy area on the west bank of the Tiber across from the city of Rome, located between the Janiculum, the Vatican Hill and Monte Mario, down to the Aventine Hill and up to the confluence of the Cremera creek. The territory of Vatican City is part of the Vatican Hill, and of the adjacent former Vatican Fields. It is in this territory that St. Peter's Basilica, the Apostolic Palace, the Sistine Chapel, and museums were built, along with various other buildings. The area was part of the Roman "rione" of Borgo until 1929. Being separated from the city, on the west bank of the river Tiber, the area was an outcrop of the city that was protected by being included within the walls of Leo IV (847–855), and later expanded by the current fortification walls, built under Paul III (1534–1549), Pius IV (1559–1565), and Urban VIII (1623–1644).