Also known as Fortress of Saint Barbara or Swabian Castle, the Emperor’s Castle was built by order of Frederick II of Swabia around the mid 13th century, and is a unique example of its kind in central-northern Italy. The building’s rigorous geometric plan and the construction technique using carefully squared blocks of alberese (a type of limestone), recall examples of Swabian military architecture in Southern Italy. This is confirmed by sources which recall that the Sicilian architect Riccardo da Lentini had the workers needed to build the castle summoned from Puglia. Conceived as the seat of the Imperial Vicar in Tuscany, the interior of the castle was never completed. Several capitals sculpted on the internal face of the walls are the only evidence of the buildings which were supposed to be erected in the square courtyard of the complex. Worthy of note is the careful placement of embrasures, conceived to assure archers and crossbowmen the possibility of front and side shots. The only concession to local architectural traditions is the entrance, marked by the green and white marble, typical of the Tuscan Romanesque style.