Everybody thinks of the Vatican as the ancient seat of the Popes, but that is not actually the case. The Cathedral of the Bishop of Rome is the great Basilica of St John Lateran, which dates from the time of the Emperor Constantine, under whose reign the legal repression of Christianity was lifted. The Vatican itself is the site of the burial (and probably also the crucifixion) of the apostle St Peter, and the same Constantine had a vast Church built there too. From 1870 onwards, following the loss of the Papal States, the Popes resided permanently in the Vatican (Pius IX described himself as a "Prisoner in the Vatican”) and the situation remained fluid until 1929 when, by an agreement with the Italian government, the Vatican State came into formal existence as a separate and sovereign entity.
The marvellous basilica of St Peter was rebuilt progressively throughout the 16th century and beyond, to replace the ruinous earlier building dating from Constantine’s time. The names of Michelangelo, Pope Julius II and Bramante stand out most prominently among those responsible for the huge project. The dome was a particular focus for attention, Michelangelo's design replacing Bramante’s, though it is interesting to note that its width did not quite equal that of the ancient Pantheon!
The Vatican Museums were founded by Julius II but developed by a sequence of subsequent scholar popes whose varying interests are reflected in the wide and eclectic range of the holdings. It ranks, without question, as one of the world's greatest museums. Included within it is the Vatican Library, founded in 1475, which holds probably the world's largest collection of manuscripts (i.e books written by hand, not printed). It has always been a semi-public library, whose resources have been made available to scholars of all faiths.
A visit to the Vatican City and St Peters can easily be incorporated into a wider visit of Rome and the rest of Italy.