Only example of Swabian architecture in central-northern Italy, it was built by the Sicilian architect Riccardo da Lentini between 1237 and 1248 and commissioned by the emperor Frederick II of Swabia.
The castle has a square floor plan strengthened at the corners by great towers, also with a square floor-plan. The alberese structure, is crowned by Ghibelline "dovetail" battlements restored in 1933. Frederick II never arrived in Prato, however his vicar in Tuscany, in charge of guarding the road that connected the Holy Roman Empire with southern Italy and Sicily through the Apennine pass of Montepiano and the Bisenzio valley, lived in the fortress. Emperial symbols are the lions of Swabia carved on the sides of the access portal, in green serpentine and alberese stone, as well as the classical tympanum reminds the imperial power of Rome in Caesar time. During the fourteenth century the Florentine government transformed the building into military garrison and prison, connecting it to the fourteen century walls through a covered walkway: the Keep.
From the inside you access the large courtyard and the rooms of the towers. Stairs inside the great tower on the eastern corner are used to reach the upper walkway that runs along three sides of the perimeter. From here one can enjoy a superb panorama overlooking the city monuments and the hills and surrounding plane. Inside the towers there are interesting works of the contemporary artist Salvatore Cipolla that remind, as symbols, the period of Frederick II. In summer the castle becomes a space for outdoor performances, concerts and movie projections.