Ave Roma Immortails, Volume 2 by F. Marion Crawford

The Mons Vaticanus is sometimes said to have received its name from 'vaticinium,' an oracle or prophecy; for tradition says that Numa chose the Vatican hill as a sacred place from which to declare to the people the messages he received from the gods. It is not, however, one of the seven hills on which ancient Rome was built, but forms a part of a ridge beginning with the Janiculum and ending with Monte Mario, all of which was outside the ancient limits of the city. In our day the name is applied only to the immense pontifical palace adjacent to, and connected with, the basilica of Saint Peter's.

The present existence of this palace is principally due to Nicholas the Fifth, the builder pope, whose gigantic scheme would startle a modern architect. His plan was to build the Church of Saint Peter's as a starting point, and then to construct one vast central 'habitat' for the papal administration, covering the whole of what is called the Borgo, from the Castle of Sant' Angelo to the cathedral. In ancient times a portico, or covered way supported on columns, led from the bridge to the church, and it was probably from this real structure that Nicholas began his imaginary one, only a small part of which was ever completed. That small portion alone comprises the basilica and the Vatican Palace, which together form by far the greatest continuous mass of buildings in the world. The Colosseum is 195 yards long by 156 broad, including the thickness of the walls.