The obelisk from Heliopolis, Egypt was brought to Rome by Emperor Caligula in 37 CE. It originally stood in his circus on a spot to the south of the basilica, close to the present Sacristy. Sixtus V had Domenico Fontana move it in 1586 to the center of St. Peter's Square. It stands 25.31m high and weighs 330 tons.

In transforming the small existing piazza, Bernini was expected to cater for several functions. "Four rows of simple and majestic Doric columns (300 in total) all carved from Roman travertine. This forms an oval 650 feet across the long axis marked by three monuments: positioned laterally by fountains propelling tall jets of water, and in the center, by an Egyptian obelisk that had served as a turning post in the chariot races at the ancient Circus of Nero. As they enter the piazza, the faithful are embraced by "the motherly arms of the church," Bernini's own description of his Colonnade. The Colonnade becomes simultaneously a dramatic frame for the church, a nurturing enclosure for the crowds of faithful, and a stage for the processions and other sacred spectacles on which, at this particular period, the Catholic Church so strongly depended for its appeal." p. 343. Ninety-six statues of saints and martyrs are atop the balustrade.