The Rocca degli Aldobrandeschi marked the history of Piancastagnaio since the origin of its medieval settlement. The information on the "Castrum Plani Castagnaie" dates to the XIII century, when the Aldobrandeschi raised the fortress in the domain of their policy of control and expansion over the Grosseto's mountain.

At the beginning of its construction, the Rocca degli Aldobrandeschi was a castle. Its highest tower, called the "Rivellino", overlooked the seat of the municipality and a long wall with two squared keeps embraced a vast area of the territory. During the years, the fortress had different rulers and in 1303 the Republic of Orvieto owned it and created a big military garrison here.
In 1345, the Rocca was seized by the Sienese who destroyed its walls. Despite its inhabitants' strenuous resistance, the castle became a dominion of the Republic of Siena. Only in 1416, after the concession of the statutes to the town, the Sienese seigniory sent two military engineers to repair the castle previously damaged. Nevertheless, the most meaningful works were carried out between 1471 and 1478. The lower part of the building and its walls were reinforced by deeply changing the whole structure. The Sienese emblem was put above the new donjon.
During the Medicean domination, when Piancastagnaio was a feud of the Marquis Bourbon del Monte, the Rocca became a prison. With the arrival of the Lorraine and with their reforms, that completely put an end to feudalism in Tuscany, the fortress, that had already lost its military importance for a long time, was no longer used. It seemed that its decadence could not be stopped until 1962, when a member of the Liguria's section of the Istituto Italiano dei Castelli bought the building and provided for repair works that gave it its ancient magnificence back.