This Castle was one of the innumerable existing in the Middle Ages of which the Guidi Counts were owners for a long time. Castel Castagnaio had a military function, it was a fortified outpost that made it possible to control the nearby possessions, as the high and perched position favored the sighting of all the other settlements belonging to the Guidi such as Porciano, Stia, Papiano. The castle was built on the foundations of a Roman temple, built in turn on the walls of an older one. Today the remains of the castle are surrounded by support structures to prevent the collapse of what remained of the suggestive ruin.