Plans to build the castle started with Cosimo I de' Medici in 1519 and the completion of the structure happened under the rule of Alessandro de' Medici in 1534. Originally, there was the castle built by the Pisans in 1377 called Quadratura dei Pisani probably built on the ruins of an ancient Roman castrum.
On 19 March 1606 Ferdinando I proclaimed Livorno a "city" in a ceremony which took place inside the fortress. It features a pentagonal layout, with imposing walls in brick and is surrounded by the Medici moat (Fosso Reale). There is a canal system, which was built to connect the two forts.
The New Fortress represents the old "Baluarte di San Francesco", designed by Bernardo Buontalenti. The construction of the new fortress dates from the late sixteenth century, when the Medici commissioned the architect to devise a plan for the new city of Livorno.