The fortifications were commissioned by Cosimo I de' Medici, after the conquest of the Republic of Siena and its annexation to the Grand Duchy of Tuscany.

The walls remain largely intact and are fully walkable, except for a brief section to the north where Porta Nuova was originally located.

After the annexation of the Republic of Siena into the Grand Duchy of Tuscany in the mid-16th century, Cosimo I de' Medici commissioned engineer Baldassarre Lanci to design and build a new city wall.

The new hexagonal walls were equipped with substantial defensive bastions at the corners, mostly in a pentagonal shape, each featuring guard posts―known as "garitte" or "casini"―at the outermost points.

Between 1939 and 1941, the Fascist municipal administration demolished a short section of the walls in the Porta Nuova area to expand the Casa del Fascio.