The Metropolitan Museum of Art guide

The Vatican is a unique domain. The worlds smallest independent city state, it is the seat of the papacy—and thus the spiritual headquarters of one of the worlds great religions, Roman Catholicism—and at the same time it is among the largest and most distinguished complexes of architectural and artistic treasures in the West.
To the ancient Romans, the Vaticanum was a swampy and ill-defined region along the bank of the Tiber. It was the site of the Vatican Circus—begun by Caligula and finished by Nero—and the place where the apostle Peter, the first bishop of Rome, was crucified and buried. Less than three centuries later, the Holy Roman emperor Constantine erected a monumental basilica over the site of Peters tomb, a church that stood for more than twelve hundred years until it was torn down, beginning in the 1500s, to make way for the magnificent Renaissance basilica that is today the focal point of Christendom.