The huge Fortress, pentagonal in shape, is the greatest work of modern fortification inserted in fourteenth century walls of Florence. Born as "Alessandria Castle", it was built by Pier Francesco da Viterbo and Antonio da Sangallo between 1534 and 1537, by order of Alessandro de' Fedici, to ensure control of the city and provide shelter to the troops. On the side facing the city, it was given a monumental aspect. The role of the military Fortress was maintained even in the Lorraine period, when they were built other buildings of architectural interest and environmental, as the House for the Officers and a Theater. The Fortress, that like many other similar was never used for military purposes, was entrusted to the State until 1967. In recent years has been identified as the site of the Florence Fairgrounds, then restored and used as trade fairs and cultural events. Cortina and Ramparts were built in brick; the Mastio, in pietraforte, and the walls were carved with a decorative pattern on balls and diamonds, alluding to the Medici coat of arms. Within the Platform you can visit the Octagon Room, equipped with an extraordinary vaulted brick herringbone, which was part of the access path to the monumental Fortress.