The current fortress is a massive polygonal contruction included perfectly in the town walls. It was built under the direction of Antonio da Sangallo (the Younger) and Nanni Unghero between 1538 and 1560 on the site of the old medieval cittadella, which had been razed to the ground because it was in the way of the line of fire of the cannons, and englobed most of the trapezoid Forte designed by Giuliano and Antonio (the Elder) da Sangallo at the beginning of the 16th century.
The bulwarks of the east side (those of the Ponte di Soccorso and the Chiesa, which are recognizable because of their sticking out shape) and a few stretches of curtain are left from the older construction. The bastions of the west side (known as the Belvedere, Spina and Diacciaia), and the interior, consisting of an intricate network of by and large unpracticable rooms, tunnels, wells and air inlets situated at varius levels, belong to the new part.
The Fortress, which originally had three gates and was surrounded by a wide moat, was in working order until the late eighteenth century. In the year 1800, it was partially dismantled by the French troops; on the west side we can still see the damage caused by a powerful explosive.
The wide and panoramic view from the glacis dominates the town, the Aretine plain, the Arno valley, the Pratomagno massif, the Alpe di Catenaia and peaks of Poti and Lignano.