After the incursion of Fra Moriale in 1354, who took up a defensive position in the town of San Casciano, refusing to go until he was paid 16,000 gold florins, the Republic of Florence decided to fortify the whole town.
The walls were 7 metres high and 2,135 Florentine “braccia” (1,240 metres) long, with four city gates: two main gates (Porta Fiorentina and Porta Senese) preceded by outer gates, and two minor ones (Porta Empolese and Porta del Prato).
In 1356 the stronghold was erected, a fortress providing refuge inside the walls, which was also used to host important guests. The walls, in the shape of an irregular hexagon, are all still standing except for those along Viale Corsini.
Of the four gates, the only surviving one is Porta del Prato, also called the Porticciola, although it was rebuilt after the Second World War. The others were demolished in the 18th and 19th centuries, so that only the great tower of Porta Senese remains today.
The best preserved side of the walls is found along Piazza del Mercato, where a work by Mario Merz was placed in the late 20th century.