The Castello dell'Imperatore is the most unexpected sight in Prato: a square, crenellated fortress rising from the city centre, built by the Holy Roman Emperor Frederick II in the 1240s. It is the only medieval imperial castle on the Italian peninsula north of Rome, and its Sicilian Norman design — angular towers, squared battlements, rigid symmetry — looks nothing like the Tuscan architecture surrounding it.
Frederick built it to serve as a garrison and stopping point on his route between Germany and his Sicilian kingdom. After his death in 1250 it changed hands repeatedly, serving as a prison and later as a Habsburg military depot. Today you can walk the ramparts and look out over the city and the surrounding plain; the interior hosts occasional exhibitions. The views are best in the late afternoon.