The fort of Petriolo was located at the northern limit of Civitella Paganico, at the border of the Borough of Monticano.
Again it was associated with the Ardengheschi family and was well known because of the curative properties of its sulphur water. The location has a long reputation and was celebrated by the writer and poet Folgore da San Gimignano.
While Petriolo was known to the Etruscans and Romans, it is also mentioned in a document of 1130. Little-by-little its reputation spread, not only within the immediate area but also across Italy and even abroad, and was visited by many well-known people. The most famous visit, in particular with regard to the history of the Republic of Siena, was that of Pope Pius II Piccolomini who made a number of visits in order to cure the gout from which he suffered. Indeed a number of important Papal Bulls were issued from this region.
The Republic had a number of single cabins built in 1250 that were rented to bathers. They also carried out research in order to open new thermal springs and farm buildings in 1330, as well as establishing a vicarship. Ten years later they initiated a tax on the bathers.
In the seventeenth century the area began to decline in favour of the baths at Pignoni and San Filippo. However the Ospedale di Santa Maria della Scala in Siena revived the springs which came under the authority of the Grand Duke of Tuscany, who passed it on to the Cospi family in 1648.
In 1713 the Ospedale obtained permission to demolish the castle walls and to make use of its stonework.
Today all that remains of the walls are those connected to the tower.
Only one of the four original baths remains. The building of a luxurious hotel and a thermal health spa with all modern conveniences will be an important factor in renewing the reputation of the Petriolo thermal baths.