On 16th July 1406 the Florentine army succeeded in conquering Vicopisano after a nine months siege. In defence of the precious conquest it was necessary to build a fortress, that ought to be impregnable. That’s why the Government of Florence commissioned to Filippo of Ser Brunellesco (Filippo Brunelleschi) a project that caused a sensation since the presentation of its wood and clay model to the Government Commission. A member of that Commission was the young leader Francesco Sforza, who would later become the Duke of Milan.
The building of the fortress was started in 1435: a great number of the churches and palaces that existed on the top of Vico hill were demolished. The Brunelleschi fortification englobed an ancient tower dating back to the 12th century, once a possession of the Archbishop of Pisa: the Tower of Santa Maria, that became the donjon of the fortress. The typology of the Brunelleschi’s fortress is medieval: high walls with battlement resting on little arches, and loopholes which allowed the pouring of burning rosin and boiling oil on the assailers.
What was really remarkable, however, was the ingenious and innovative system of drawbridges, that, when lifted, could cut off parts of the fortress from the rest, in the event of an invasion by enemies. For instance, before entering the courtyard of the Rocca, one had first to break through the ante-door with its drawbridge and moat. In case of impending loss of the courtyard, the defendants could demolish the stair - based on four slender arches – that joined the courtyard to the patrol route on the curtains. In the event the enemy had succeeded in reaching the curtains, the defendants would consolidate a bridgehead in the tower: by retrieving the drawbridge that joined the patrol route to the only entrance of the tower it was possible to isolate the tower from the rest of the fortification. The tower was provided with a cistern and a depot to store up provisions that allowed to withstand long sieges.
The most ingenious solution was however the powerful battlemented massive wall that descended from the Rocca to the foot of the hill, ending in a 21 meters high tower, the “Torre del Soccorso”, which was situated at that time near the river Arno: if the enemy had entered Vicopisano, it would be impossible for them to besiege the Rocca and cut off the supplying of food. 
Who tried to attack the massive wall would have been mown down by the fire of the culverins coming from the Rocca and by the artillery positioned on the gun ports of the Tower, while light fire and crossbow shooting would have come from the massive wall battlements. From the Torre del Soccorso (Defence Tower) – positioned near the Arno river – it was possible to get support from Florence: the boats could dock at a little cove defended by fortifications (no more existing), unload gun powder and men that, through a narrow portal and by means of ladders, reached the second floor of the tower. From that position they were able to access the massive wall and go up towards the Rocca.
In the event of the seizure of the “Torre del Soccorso”, it would be very hard for the enemy to cover the battlemented passageway exposed to the fire coming from the Rocca; moreover, they would find the only possible connection between the massive wall and the patrol route- a door with a drawbridge - interrupted: one can still observe the three meters jump and fifteen meters drop.
Today, from the tower once used for signalling to Florence, one can enjoy a view on the hills and fields and relish the peaceful and serene atmosphere.
(Translated from: Le fortificazioni di Vico Pisano: un capolavoro dell'ingegno di Filippo Brunelleschi / by G. Ranieri Fascetti - Pisa; ETS, 1998)