The ring of walls, today a bit less than 3 kilometers, was constructed by the Etruscans around the end of the 5th century B.C. and can be seen at the base of the current day walls. It has a rectangular form with the two longer sides facing North and South and the shorter sides facing East and West. The short west side is very well preserved and the site of a recent reopening and restoration of the Etruscan gateway with unique double barrel-vaulted portals, which belongs to the Etruscan perimeter. The first part of the larger north side from Porta Santa Maria follows the course of the Etruscan perimeter until the area above Porta Colonia where it reveals a sewer spout which dates back to the time of construction of the original wall.
The remainders of the Northern length of the wall, as well as the shorter East side, were moved slightly toward the direction of a place called Torre Mozza. There are consistent traces and evidence of this old perimeter wall and more about this place still remains to be discovered. Probably there was a temple or a fortification and an entry gate to the city (Porta di Montagna or Augurata?) at this point. The number and the positions of the Etruscan gateways are still to be determined.
In the 3rd century after the Roman conquest, the damaged parts of the walls were reconstructed and raised in height. Four gateways were placed at the exits of the Roman city: from the decumano (the principal East-West Roman road), Porta Santa Maria and Porta San Domenico (also called Peccioverardi) and from the cardo (principal North-South Roman axis road), Porta Sant'Agostino and Porta Colonia. In the Middle Ages the walls were heavily damaged during the attack of 1258, they were subsequently reconstructed and restored with the help of the people from Perugia and most of all of the Sienese. The two-opening Etruscan gateway was closed (Porta Bacarelli) and other gateways were opened: Porta Montanina (originally called San Cristoforo), Porta Berarda and Porta San Giorgio. The last two were closed-up probably at the beginning of the 17th century. In 1642, the period of the war for the Dukedom of Castro, the walls on the West side were further reinforced under the grand ducal government, as we are informed by an inscription in marble placed above the archway of Porta Bifora. Towards the end of the 19th century the existing wall was opened to form Porta Santa Margherita to give access to the sanctuary nearby.
In the second half of the 1500's, precisely in 1556, on the north east corner of the rectangular city walls, at its highest point, the first Grand Duke of Tuscany, Cosimo I had a fortress constructed. It rose from structures which had been fortifications during Etruscan, Roman and Medieval times. The last fortress was destroyed in the sack of 1258. The fortress was the work of Gabrio Serbelloni, nephew of Pope Pius IV, and of Francesco Laparelli of Cortona. The structure is trapezoidal in shape with four large bastions. It continues to be a beautiful example of military architecture of the 1500's. After its reconstruction in modern times there is no news of battles which took place in it. Recently restored, it is now the site of art exhibitions and the "Centro Studi e Documentazioni sulla Civiltà Contadina della Valdichiana e Trasimeno", a research center dedicated to the study of the peasant and folk history of the Valdichiana and Lake Trasimene areas.