The Castello dell'Imperatore - the imperial castle in Prato - is the only example of Hohenstaufen architecture in northern and central Italy. It was built between 1237 and 1248 by Riccardo da Lentini, a Sicilian architect, on the orders of Emperor Frederick II, who never reached Prato: The castle was inhabited by his bailiff in Tuscany, who was charged with protecting the road that connected the Holy Roman Empire with the south of Italy and with Sicily.
Both the imperial symbols, the Swabian lions carved into the green serpentine and Alberese on the side of the access gate, and the classical tympanum recall imperial power in the Rome of the Caesars.
From:
https://www.visittuscany.com/de/attraktionen/das-castello-dellimperatore-in-prato/