The fulcrum of the Arcidosso Museum Complex is the Aldobrandesca Fortress, which has maintained its public function over the course of its eleven centuries of life, making it a rare and precious example in the Italian and European panorama, it is one of the best preserved and oldest medieval castles in Italy and Europe (10th century) and one of the few completely accessible to the public, a suggestive container of valuable collections, inside there are four museums.

The first certain news of the existence of a settlement in Arcidosso dates back to the year 860 and reports it as a possession of the monks of the Abbey of Abbadia San Salvatore. Recent archaeological studies have established that, when around 1100 the Aldobrandeschi decided to build the main tower, the fortress already possessed a two-storey stone palace most likely built by the Marquis between 970 and 995.

The construction of the first nucleus of the Castle around which the town will develop can therefore be dated around the year one thousand. From the 11th to the 14th century it was a fief of the Aldobrandeschi counts, who transformed and expanded it by raising the palace by two floors, building its towers and the two crenellated walls, also equipped with towers; it became an important military stronghold, an extreme bulwark against the penetration of Siena into the Amiata area, becoming part of the possessions of the powerful family, whose dominion lasted until 1331 when the castle was besieged and conquered by the Sienese. Guidoriccio da Fogliano in 1331 besieged it for four months with a Sienese army of 4000 infantry and 400 cavalry, obtaining its surrender.

In the second half of the sixteenth century, following the definitive fall of Siena, the place was incorporated into the Grand Duchy of Tuscany and passed under the Medici state, Cosimo I established many peripheral offices in Arcidosso, making it an important centre. Following the Leopoldina reform of 1786, which redistributed state-owned and ecclesiastical lands, Arcidosso passed under the Lorena family. This generated a conspicuous demographic increase and its population reached 12,000 (almost three times the current one).

Today the Castle is the seat of four museums, it is used to host events, exhibitions and civil ceremonies. Inside the medieval halls :

- Museum of David Lazzaretti, retraces the human and religious story of David Lazzaretti (1834-1878), the "prophet of Amiata" and his movement, through panels, objects and documents.

- Emo Mecheroni Weapons Collection, a collection of ancient weapons, from the Middle Ages to the 1900s, different types of sidearms and firearms, rifles, animated sticks.

-Medieval Landscape Museum, an archaeological-artistic itinerary to discover the medieval fortifications in Amiata and Maremma. -Aldobrandesca Tower, which can be accessed up to the top, 28 meters high, and enjoy the 360° panorama

-Maco, is the Museum of Oriental Art and Culture, built thanks to the Dzogchen community of Merigar, which has been present on Mount Amiata for over 40 years.

- Torre Aldobrandesca: it is possible to climb the tower, 28 meters high, and enjoy the 360° panorama