The fountain, remaining unchanged, until the late 18th century when change seemed to be looming on the horizon! There was a recording on a small piece of paper written by Pope Alexander VII that was a grand new scheme for the area of Monte Cavallo. Alexander kept a model of Rome so he would be aware of all of the projects going on. This paper marked the change to the Spring of 1667. This scheme describes the following: "This fountain at Monte Cavallo to Piazza dei Santi Apostoli. The Spire at Campo Marzo to be erected here at Monte Cavallo. Take the horses to this and that side of the gateway with water as sketched, cutting off the corner of that wall. Refinish Porta Pia so that the said water is facing it."
In regards to the fountain, Bernini had plans to replace the fountain with one of his own designs that would be similar to the existing one, as is evident with the drawing.
The second of the two obelisks, is what the “Spire” at Campo Marzio refers to as, the one that was once lying near the Mausoleum of Augustus, ever since the ancient Roman time. Bernini’s idea was finally implemented in the end of the 18th century.
The final part of the note is difficult to interpret, because it was so short. It seems to relate to a specific terminus for Strada Pia where it reaches Monte Cavallo. This involved the construction of a gigantic arch that would be the home, in one of the pillars, of the cistern for the Aqua Felice that Sixtus V had built in the back of the Castor and Pollux statues. The top of the arch would have the crest on one side, the hills and the stat, of Alexander VII. The statues of Castor and Pollux would be placed on either side of the arch with the fountain in bewteen. They would no longer be in line now, but facing different directions.
Bernini’s intent was clear, it was to create a counterpart to Michaelangelo’s monumental archway at Porta Pia, located at the opposite end of the street.
