Castle began to be built around year 860 by Aldobrandeschi family, on existing buildings of Lombard period. It is the oldest suburban palace of government in Italy and one of the oldest in Europe. Aldobrandeschi transformed and expanded the Castle by raising two-storey building towers and crenellated town walls. The last works were carried out by Republic of Siena after 1332. The building consists of two buildings characterized by a section resting on huge bases in the shoe. The northern side is characterized by the presence of a main tower that rises beyond the roof, the top of which is crowned by blind arches.