The free thermal baths of Petriolo are the most famous free baths in Tuscany after Saturnia, and probably in earlier times were also much more prestigious than the latter. At present, the baths built along the course of the Farma river limit themselves to capturing the excess water not used by the very recent paying spa built a few metres away, whereas previously they boasted an abundant flow of very hot water. As can still be seen from the brick ruins located near the thermal spring, Petriolo represents a very rare case of "fortified baths" in history. The residual stumps of walls belong to the ancient "Castle of Petriolo", a defensive structure built to protect the spring and the baths, which were considered so precious that they could not be left at the mercy of threats from the various potentates in the area. The reputation of its waters is testified by the visit of Pope Pius II for curative purposes, who came here to heal the "podagra", a disease from which he suffered and which prevented him from walking without pain. The Republic of Siena made people pay a tax on the use of the Petriolo baths, an almost unique case among the various free Italian spas, and it too testifies to the extraordinary value attributed to these medicinal waters.
