The Great Piazza of St. Peter's
The final piece of St. Peter's that was needed to complete the basilica was public space in front of it where large crowds could assemble and be blessed by the Pope either from the basilica or from a window in the Vatican palace.
In the last decades of the 16th century, the stage had been set for a new piazza by a project executed under Pope Sixtus IV and Domenico Fontana, one of the most gifted engineers of his day. In an effort to put order into the more or less chaotic organization of Rome, especially to open up areas of the city outside its center for repopulating them, Sixtus IVasked Fontana to design new axial avenues connecting major ancient and early Christian monuments. The system of streets determined the principal order of Rome until the 20th century. Part of its success was Fontana's decision to punctuate the new streets with obelisks moved from other locations to critical points along the streets. Since the main trafric was pedestrian, the obelisks formed a human mensuration system by means of which a pedestrian could judge where and how far he or she was going and how much progress had been made along the way.
The use of the obelisk as the focus of public space was actually reintroduced by Sixtus IV and Fontana at St. Peter's when about 1585, Fontana moved an obelisk from the south side of St. Peter's to a location in front of the ancient basilica.
In the distance, you can see the drum of the Michelangelo and della Porta's dome going up.
Using the obelisk as the reference point, Bernini added two fountains on the cross axis and created an oval space with the obelisk and fountains as foci. He then enclosed the oval on three side with a double colonnade. (The third piece was on the bottom or east side and has since been removed.) This large oval area is connected to a trapezoid in front of Maderno's facade and opens up to the facade similar to giant pincers.
The Tuscan Doric columns of the colonnade subordinate themselves to the facade of St. Peter's both in kind (the facade uses the Corinthian order) and in size (the giant order of the facade is about twice the height of the colonnade). This gives an additional sense of verticality to the facade.
Finally, as in all of Bernini's work, the architectural validity of the project is infused with symbolism. Bernini said that he envisioned the colonnade in an anthropomorphic way, as though the arms of the Mother Church were reaching out to embrace all of humanity. As with Michelangelo, Bernini's genius as a sculptor not only informed his other art but ultimately needed the scale and power of architecture in order to come to full expression.