Initial construction of the Prato city walls began in 1175 and was completed in 1196, fitting the city with a wall made of square-block alberese stone, complete with towers, bartizans and eight gates.
The first outer city walls in Prato were built in a four-cornered shape. According to a Medieval expert, initially gates along the waterways were built adjacent to the bridges before the rest of the wall was erected and connected the existing gates together.
Following expansion of the suburb area outside the second city walls, it was necessary to invest in new structures. This resulted in the construction of the first section of the wall found in the Mercatale zone. The next section extended up to porta Gualdimare, construction work was completed in 1332. Construction to both sites probably took place independently from the two extremities of the wall that ran along the Bisenzio river, building along the northern side up to porta Gualdimare and from the west side up to Rocca Nuova.
Between 1338 and 1351 the walls between Porta Gualdimare, Porta S. Trinita and the fortress or cassero were constructed. Following the outbreak of plague in 1348, construction of the remaining sections was halted by the city council however construction restarted again in 1382 by order of Florence city council (under which Prato was governed).
The Corridore del Cassero, or “corridor of the castle”, was requested by the city of Florence to be built immediately following the subjugation of Prato under Florentine jurisdiction in order to connect the small fortress to the city walls via the east gate. The top crenelated wall is formed of a barrel-vaulted passage built into the immense wall and fitted with a long run of lunette windows cut in with small rectangular windows that allow light into the passage. Another hidden walkway runs along the upper part.
The interior layout of the city was originally defined by the main entrances of the periphery walls. The eight gates (Porta San Giovanni, Porta Tiezi, Porta Capo di Ponte, Porta Corte, Porta Santa Trinita, Porta Fuia, Porta Gualdimare and Porta al Travaglio) divided the land into eight districts (also identifying the district names). At the time, this was an unconventional decision for the urban landscape of Tuscan towns and cities was typically divided into three, for example in Siena, Volterra and Pisa, into six, as in Florence from the 12th century up to 1343, or even four, as in Pistoia and Arezzo. However, in order to facilitate local administration, the city council eventually chose to consolidate the eight gates into pairs.
The “eight gates” then became the “district gates”, and are as follows: