Rightly [did] Pope Benedict XVI speak of this basilica as the heart of the Roman Church, as St. John Lateran is the head. It is here that the Church honors her first shepherd in this city, and here that since his martyrdom she has celebrated both his witness and the God he served. While the basilica before us is relatively modern as far as the history of Christianity goes, being completed only in 1626, Christians have been coming to this site to ask for his intercession since shortly after the death of the Prince of the Apostles, as messages left by them on the wall of his grave attest.
The first structure on this site to concern us was a circus built by the Emperor Caligula around the year A.D. 40. It was here that the leader of the Apostles was crucified upside down at his own request, so as to die in a way yet more humiliating than that of his master. Soon after, his simple tomb in the style of a poor man’s grave was constructed by leaning tiles together in the shape of a tent over his shallow grave. This was soon succeeded by a small shrine, which was the focus of devotion here until the legalization of Christianity under Constantine. As part of this emperor’s building program on behalf of the Church, a large basilica was built on the side of the Vatican hill. This required much moving of earth to prepare the surface so that the basilica could be built directly over the saint’s grave. The building of this first basilica of St. Peter began in about 320, and was completed before the close of the decade. At first it mainly served as a covered cemetery in which Christians could be buried near the great saint, although Eucharistic celebrations would be occasionally held on a temporary altar near the shrine of the saint. In the late sixth century the first permanent altar was built over the tomb of St. Peter, around the same time that the sanctuary was greatly enlarged and remodeled. In the succeeding centuries the basilica was repeatedly embellished and ravished, yet remaining all the while one of the most important churches in Christendom. In the fifteenth century began the first stirrings of a plan to replace this church with one more telling of the grandeur of the newly revitalized Rome during the Renaissance. In 1506, Pope Julius II laid the cornerstone for the current basilica, and over the following 120 years construction continued in fits and starts. When it began, Europe was united under the authority of the Pope, and the New World had just been discovered by Columbus. By the time of its dedication Europe was divided by wars of religion between Catholic and Protestant, and the settlement of the English colonies in North America was well underway. Several famous architects contributed to the design of the building, among them Bramante, Raphael, Michelangelo, and Maderno. Even after the dedication of the basilica on 18 November 1626, the work continued. Much of this took place under Bernini, who is responsible for the ciborium, the sculptural group in the apse, and the colonnade surrounding St. Peter’s Square in front of the church. Since the return of the papacy from Avignon in 1376, the pope has normally maintained his residence here, with the result that this is the location of most of the major papal liturgies throughout the year. It is also the setting for extraordinary events, such as the election of new successors to St. Peter.
There are SO MANY things of interest inside the basilica that, rather than trying to describe them, I invite you to take a "virtual tour" via the official website for the Basilica.