Vatican, St. Peter's Square, granite fountain by Carlo Maderno - just after a thunderstorm.

St. Peter's Square, Rome, southern view, western colonnades to right, granite fountain by Carlo Maderno - just after a thunderstorm.

St. Peter's Square (Italian: Piazza San Pietro, Latin: Forum Sancti Petri) is a large plaza located directly in front of St. Peter's Basilica in the Vatican City, the papal enclave inside Rome, directly west of the neighbourhood or rione of Borgo.
Gian Lorenzo Bernini designed the square, including the massive Tuscan colonnades, four columns deep, which embrace visitors in "the maternal arms of Mother Church." A granite fountain constructed by Bernini in 1675 matches another fountain designed by Carlo Maderno in 1613.

The colossal Tuscan colonnades, four columns deep, frame the trapezoidal entrance to the basilica and the massive elliptical area which precedes it. The ovato tondo's long axis, parallel to the basilica's façade, creates a pause in the sequence of forward movements that is characteristic of a Baroque monumental approach. The colonnades define the piazza. The elliptical center of the piazza, which contrasts with the trapezoidal entrance, encloses the visitor with "the maternal arms of Mother Church" in Bernini's expression. On the south side, the colonnades define and formalize the space, with the Barberini Gardens still rising to a skyline of umbrella pines. On the north side, the colonnade masks an assortment of Vatican structures; the upper stories of the Vatican Palace rise above.

Carlo Maderno (1556 – 30 January 1629) was an Italian architect, born in today's Ticino, who is remembered as one of the fathers of Baroque architecture. His façades of Santa Susanna, St. Peter's Basilica and Sant'Andrea della Valle were of key importance in the evolution of the Italian Baroque. He is often referred to as the brother of sculptor Stefano Maderno, but this is not universally agreed upon.

Gian Lorenzo Bernini (also Gianlorenzo or Giovanni Lorenzo; 7 December 1598 – 28 November 1680) was an Italian sculptor and architect. While a major figure in the world of architecture, he was the leading sculptor of his age, credited with creating the Baroque style of sculpture. As one scholar has commented, "What Shakespeare is to drama, Bernini may be to sculpture: the first pan-European sculptor whose name is instantaneously identifiable with a particular manner and vision, and whose influence was inordinately powerful...." In addition, he was a painter (mostly small canvases in oil) and a man of the theater: he wrote, directed and acted in plays (mostly Carnival satires), also designing stage sets and theatrical machinery, as well as a wide variety of decorative art objects including lamps, tables, mirrors, and even coaches. As architect and city planner, he designed both secular buildings and churches and chapels, as well as massive works combining both architecture and sculpture, especially elaborate public fountains and funerary monuments and a whole series of temporary structures (in stucco and wood) for funerals and festivals.

The open space of St. Peter's Square which lies before the basilica was redesigned by Gian Lorenzo Bernini from 1656 to 1667, under the direction of Pope Alexander VII, as an appropriate forecourt, designed "so that the greatest number of people could see the Pope give his blessing, either from the middle of the façade of the church or from a window in the Vatican Palace". Bernini had been working on the interior of St. Peter's for decades; now he gave order to the space with his renowned colonnades, using the Tuscan form of Doric, the simplest order in the classical vocabulary, not to compete with the palace-like façade by Carlo Maderno, but he employed it on an unprecedented colossal scale to suit the space and evoke a sense of awe.
There were many constraints from existing structures. The massed accretions of the Vatican Palace crowded the space to the right of the basilica's façade; the structures needed to be masked without obscuring the papal apartments. The obelisk marked a centre, and a granite fountain by Maderno stood to one side: Bernini made the fountain appear to be one of the foci of the ovato tondo embraced by his colonnades and eventually matched it on the other side, in 1675, just five years before his death. The trapezoidal shape of the piazza, which creates a heightened perspective for a visitor leaving the basilica and has been praised as a masterstroke of Baroque theater, is largely a product of site constraints.

more on https://en.wikipedia.org/wiki/St._Peter's_Square , https://en.wikipedia.org/wiki/Carlo_Maderno , https://en.wikipedia.org/wiki/Gian_Lorenzo_Bernini , https://en.wikipedia.org/wiki/St._Peter's_Square#Colonnades

picture generously provided by Rudolf Kaufmann (2015)