San Pietro gets some work done, 1629
Bernini was appointed Architect of St. Peter's in 1629, following Carlo Maderno. Commissioned by Urban, Bernini would go on to renovate the facade, create the open piazza, and raise the Scala Regia. Maderno has planned to add towers on opposite ends and Bernini attempted to follow through. However, due to the instability of the ground these towers, near completion, began to crack. They were subsequently taken down. During this time Urban died and was replaced by Innocent X, who was not as supportive of Bernini's choices.
Luckily, by 1655 Alexander VII was in place and together they got the construction of the piazza started. Composed of a trapezoidal shape feeding into a large oval, all of which is surrounded by colonnades. The rows of Tuscan columns are topped with a trabeated entablature. This innovative design gave the church an illusion of greater height (countering Maderno's bottom heavy facade but did not overpower it. Bernini had included in his plan a third colonnaded section, adding an element participation. It would have allowed those passing by to see through the columns, treated to different views of the awe inspiring church. This was not added on, however.
Around 1663 Bernini started work on the Scala Regia. As you can see in the plan, there were some discrepancies in the continuity. In order to maintain the illusion of naturalism, Bernini has varied the distance between the columns and the walls and reduced diameter when needed. This use of perspective along with the Palladian motif of a broken entablature arch on columns creates a sculptural effect.