The modern wall circuit largely matches the ancient and original lower rows belonging to the first boundary wall measuring up to 14 feet in height and consisting of large superimposed stone-blocks laid in regular ashlar courses to form portions of up to nine overlapping rows.
The overall shape is that of rectangle having a perimeter of about 2 miles. Only in the south-east the modern circuit veers off the original Etruscan circuit although surveys carried out in 1929 by the Archaeological Superintendency confirmed the presence in this portion of ancient walls extending well beyond the later medieval wall circuit built in the first half of the 13th century. No trace was left of the original gates with the exception of the Porta Bifora although one may suppose they possibly matched the later medieval gates. The original boundary walls date back to the late 4th century B.C.