Old St. Peters Basilica was the building that stood, from the 4th to 16th centuries, on the spot where the new St. Peters Basilica stands today in Vatican City. Construction of the basilica, built over the site of the Circus of Nero. The name old St. Peters Basilica has been used since the construction of the current basilica to distinguish the two buildings, construction began by orders of the Roman Emperor Constantine I between 318 and 322, and took about 30 years to complete. Over the next centuries, the church gradually gained importance. Papal coronations were held at the basilica, and in 800, in 846, Saracens sacked and damaged the basilica. The raiders seem to have known about Romes extraordinary treasures, some holy – and impressive – basilicas, such as St. Peters Basilica, were outside the Aurelian walls, and thus easy targets. They were filled to overflowing with rich liturgical vessels and with jeweled reliquaries housing all of the relics recently amassed, as a result, the raiders pillaged the holy shrine. In response Pope Leo IV built the Leonine wall and rebuilt the parts of St. Peters that had been damaged, in 1099, Urban II convened a council including St Anselm. Among other topics, it repeated the bans on lay investiture, by the 15th century the church was falling into ruin. Discussions on repairing parts of the structure commenced upon the return from Avignon. The whole stretch of wall has been pierced by too many openings, as a result, the continual force of the wind has already displaced the wall more than six feet from the vertical, I have no doubt that eventually some. Slight movement will make it collapse, at first Pope Julius II had every intention of preserving the old building, but his attention soon turned toward tearing it down and building a new structure. Many people of the time were shocked by the proposal, as the building represented papal continuity going back to Peter, the original altar was to be preserved in the new structure that housed it. Constantine went to pains to build the basilica on the site of Saint Peters grave. The Vatican Hill, on the west bank of the Tiber River, was leveled. Notably, since the site was outside the boundaries of the ancient city, the exterior however, unlike earlier pagan temples, was not lavishly decorated. The church was capable of housing from 3,000 to 4,000 worshipers at one time and it consisted of five aisles, a wide central nave and two smaller aisles to each side, which were each divided by 21 marble columns, taken from earlier pagan buildings. It was over 350 feet long, built in the shape of a Latin cross, and had a roof which was timbered on the interior
19th-century drawing of St. Peter's Basilica as it is thought to have looked around 1450. The Vatican Obelisk is on the left, still standing on the spot where it was erected on the orders of the Emperor Caligula in 37 A.D.
Fresco showing cutaway view of Constantine's St. Peter's Basilica as it looked in the 4th century