Ciudad del Vaticano

The placename is ancient and predates Christianity, coming from the Latin Mons Vaticanus, Vatican Hill. It is part of the Mons Vaticanus, and of the adjacent former Vatican Fields upon which St. Peter's Basilica, the residence of the popes called the Apostolic Palace, with its Sistine Chapel, and museums were built, along with various other buildings. The area was never fully incorporated into the urban conglomeration of Rome until the last century, being separated from the city by the river Tiber. It was thus an outcrop of the city and was protected by being included in a loop of the city wall. When the 1929 Lateran Treaty that gave the state its present form was being prepared, the fact that a good part of the proposed territory was all but enclosed by this loop led to the present territorial definition being adopted. For some tracts of the frontier there was no wall, but the line of certain buildings supplied part of the boundary, and for a small part of the frontier a modern wall was constructed. The territory included St. Peter's Square, which it was not possible to isolate from the rest of Rome and therefore a largely imaginary border with Italy runs along the outer limit of the square where it touches on Piazza Pio XII and Via Paolo VI.

Although technically not included within the territory of the Vatican City State, according to the Lateran Treaties, certain properties of the Holy See have an extraterritorial status similar to that of foreign embassies. These include the papal summer residence of Castelgandolfo in the nearby hills, the Lateran Basilica, the basilicas of St. Mary Major and of St Paul Outside the Walls, and a number of other buildings in Rome. Castelgandolfo and the named basilicas are patrolled internally by police agents of the Vatican City State and not by Italian police. St. Peter's Square is ordinarily policed jointly by both.