Forte di Belvedere
The hill of Boboli is the only high ground encompassed (or partly encompassed, at least) within the late 13th century city walls, with the Gate of San Giorgio set in them at their highest point. The hills across the Arno were always considered one of the weak spots in the city's defences, a sensation that only increased with the invention of artillery in the early modern era.
The Fortezza di San Giovanni Battista (later known as the Fortezza da Basso) commissioned by Giulio de' Medici – Pope Clement VII - from Antonio da Sangallo the Younger between 1534 and 1537, was certainly intended to play a defensive role in the level area around the city, but its chief purpose was to safeguard the ducal family and court from turmoil inside the city. They could reach the fort from the nearby Palazzo Medici in Via Larga extremely rapidly.
Alessandro's successor, Cosimo I, was to have the hill of San Miniato fortified and its defensive walls erected towards the middle of the 16th century, creating a bastioned curtain wall inside the 14th century walls stretching from San Frediano to Boboli.
The grand ducal court's final move from Palazzo Vecchio to Palazzo Pitti under Ferdinando I must have influenced the decision to build the new fort right up against the walls surrounding the Boboli Gardens adjacent to Palazzo Pitti. This would allow the prince and the court to rapidly and safely reach a fortified place of shelter overlooking the city in the event of a threat coming from inside the city itself.