The construction of the Fortezza da Basso was ordered by Alessandro de’ Medici and built between 1534 and 1537 on a project by Pier Francesco da Viterbo and Antonio da Sangallo the Younger, well-known military architects of the time. Initially, it was called Castello Alessandria and only after the construction of the Belvedere Fort in 1600 did it begin to be named Fortezza da Basso (because it was located on the plain while the Forte del Belvedere was located in the hills). The reason for the construction is linked to the terrible episode of the siege of 1529-30, which caused thousands of deaths among soldiers and civilians.
 
To ensure control of the city, the Medici decided to build the military fortress that was to provide accommodation for the troops, refuge for the rulers in case of revolt, intimidate the Florentines, and extinguish any idea of​a Republic with its majesty. In the Lorraine period, other buildings were built such as the building for the officers and a theatre. During the period in which Florence was the capital of Italy, its moats were levelled and most of the walls were buried.
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Starting from 1967, a long restoration and adaptation of the Fortress was undertaken as the site of the Florentine exhibition centre. Thanks to these works, the fourteenth-century Porta a Faenza, the ancient course of the Mugnone, a stream that flowed into the city’s moats, the interior of the Keep and some other structures of some interest, such as the powder magazine, are visible today.
​Two new exhibition pavilions were created to host the Firenze Fiera events: “Spadolini” (1977) and “Cavaniglia” (1996). In total, the rooms available for exhibitions and events, in addition to the two mentioned above are: the Arsenale Pavilion, the Lorenese Theatre, the Giaie Pavilion, the Rastriglia, the Quartieri Monumentali, the Palazzina Lorenese, the Sala della Ronda, the Polveriera and the Hall of Nations.
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Inside the fortress there is one of the offices of the Opificio delle Pietre Dure with numerous scientific and restoration laboratories. The fortress was built in a pentagonal shape to insert the base side into the pre-existing Arnolfian walls. At the centre of this longer side, the so-called keep was built, which incorporates the ancient Porta Faenza which was part of the second municipal circle of the thirteenth century.