There had been a fountain at the opposite end of Santa Maria Maggiore, as was common practice. It was simple porphyry basin supported by pillars that used the Marcio aqueduct. So it only made sense that the Pope wanted a fountain to be constructed in the same square, and chose Maderno, in association with the architect Gaspare de Vecchi.
The basin, originally, was an oblong shape with eagle fountains at either end of the long ends, and dragons at the curved ends. The dragons unfortunately disappeared sometime in the 19th century, but the eagles can still be seen above the masks. It was once a wonderful sight, and these beautiful decorations created a different air to the construction, very different to the state of disrepair that exists today.
Regarding the statue of the Virgin that sits on top of the column needed 14-16 thousand pounds of metal to create image of the Holy Virgin. Pope Paul V had metal from an old canon from Castel Sant’Angelo melted down, and when this was not enough, he included “"four iron railings, each consisting of three sections and a number of arches, also in metal, which formerly surrounded the Pine Cone that stood in front of the atrium of the old St. Peter's basilica." It was removed in 1614, and the Roman saying seems appropriate, "Strip one altar to clothe another."
