Another monument absolutely to see in Florence is the Fortezza da Basso, whose official name is the Fortress of St. John the Baptist is the greatest work of modern fortification included in fourteenth-century walls of Florence. Born with the name of Castello Alessandria, it was built by Pier Francesco da Viterbo, and Antonio da Sangallo the Younger between 1534 and 1537 by order of Alessandro de ‘Medici, placed at the government of Florence by his uncle Giulio de’ Medici, Pope Clement VII who exercised the real power. In May of 1533 excavations supervised by the famous leader Alessandro Vitelli and the architect Pier Francesco Florenzuoli da Viterbo began. On 15 July 1534 the first stone was laid and in December of that year the works of fortification were completed in the majority. The giant fortress, pentagonal in shape, was built with many resources and quickly to ensure control of the city to the Medici, just returned to the city after the Siege of Florence, to provide housing to a large contingent of troops, as well as shelter to the government leaders in case of revolt, but also to impress and intimidate the Florentines with its mass. To accentuate this impression the side facing the city was provided, by Antonio da Sangallo, with a monumental appearance. The role of the military fortress was maintained even in the Lorraine period, when other buildings of architectural and environmental were built, as the building for officers and a theatre.

The fortress, which like many other similar structures was never used, was entrusted to the State Military until 1967.
In recent years he was identified as the seat of the Florentine Exhibition Center; a long work of restoration and adaptation (not yet completed) was therefore undertaken which led to the demolition of service structures built when the fortress was used as a barracks. Thanks to these works we can see today the fourteenth-century Porta a Faenza, and follow the ancient course of the Mugnone, a stream which flowed into the ditches of the city; visit the interior of the Mastio and some other structures of some interest, as the powder magazine.