The open site was a marketplace established before the thirteenth century on a sloping site near the meeting point of the three hillside communities that coalesced to form Siena: the Castellare, the San Martino and the Camollia. Siena may have had earlier Etruscan settlements, but it was not a considerable Roman settlement, and the campo does not lie on the site of a Roman forum, as is sometimes suggested. It was paved in 1349 in fishbone-patterned red brick with ten lines of travertine, which divide the piazza into nine sections, radiating from the mouth of the gavinone (the central water drain) in front of the Palazzo Pubblico. The number of divisions is held to be symbolic of the rule of The Nine (Noveschi) who laid out the campo and governed Siena at the height of its mediaeval splendour between 1292-1355. The Campo was and remains the focal point of public life in the City. From the piazza, eleven narrow shaded streets radiate into the city.
The palazzi signorili that line the square, housing the families of the Sansedoni, the Piccolomini and the Saracini etc., have unified rooflines, in contrast to earlier tower houses — emblems of communal strife — such as may still be seen not far from Siena at San Gimignano. In the statutes of Siena, civic and architectural decorum was ordered :”…it responds to the beauty of the city of Siena and to the satisfaction of almost all people of the same city that any edifices that are to be made anew anywhere along the public thoroughfares…proceed in line with the existent buildings and one building not stand out beyond another, but they shall be disposed and arranged equally so as to be of the greatest beauty for the city.”
The unity of these Late Gothic houses is effected in part by the uniformity of the bricks of which their walls are built: brick-making was a monopoly of the commune, which saw to it that standards were maintained. (Ingersoll)