The current appearance of the circle of walls, dating back to the first half of the 17th century, is the result of the many changes, which were introduced onto the Middle-Ages city walls. The works began in 1544 with the construction of the brick podium.

In 1561, thanks to the drawings of Francesco Paciotti da Urbino, the plan for the new fortification became clearer, but before Alessandro Farnese in 1589 worked out the final plan, the views about the appearance of the finished monument were very different and not well-defined.
All the engineers and architects, who oversaw the plan afterwards, drew inspiration from the work of Farnese. The city walls were completed only in 1645.
The works in the part just outside the city walls were carried out by Paolo Lipparelli and they ended five years later.
The current circle of walls with its uneven shape is 4,195 meters long and 12 meters high. The monument is made up of bricks and twelve thirty-meters bottom large curtains. S. Paolino, S. Donato, S. Croce, S. Frediano (which was never rebuilt because here the town was protect by the river), S. Martino, S. Pietro, S. Salvatore, "Libertà", S. Regolo, S. Colombano and S. Maria (which still maintains the squared shape of Pacinotti's plan) are the names of the eleven bulwarks of the city walls.
A little guardhouse was built on each bulwark.
It was used as quarters for the guards or as ammunition warehouse.

The ditch, the terreplein and twelve demilunes with a masonry podium are the impressive outer fortifications. Three were the monumental gates with drawbridges and portcullis, which led into the town: Porta S. Pietro (1566), Porta S. Maria (1593) and Porta S. Donato (1629). Today there are six gates and some other entrances, which were built only many centuries later.

When the walls were finally completed, there was no real need for them to defend the town, so they underwent some changes over the years.
The most important ones were made by Maria Luisa di Borbone, who transformed the monument into a public park by building on its top a tree-lined promenade, which is still very charming, and by Elisa Baciocchi, who let Porta Elisa to be built in 1804.
In 1840 a café was constructed on the Bulwark of S. Maria and Porta Vittorio Emanuele and Porta IV Novembre were added to the city walls in 1910 and 1930 respectively.