The Emperor's Castle is a splendid example - and the only one in central-northern Italy - of architecture of the period of the Holy Emperor Frederick II of Swabia, built after 1240 to a design by the architect Riccardo da Lentini, responsible for some of the most important castles in southern Italy of that time. The building, erected on the original site of the Alberti family castle, became the headquarters of the Emperor's vicar and hosted Frederick II's son, King Eno, and his illegitimate son Frederick of Antioch, who started off from Prato to the conquest of Florence in 1248. The castle is square in shape, with sides of about 40 metres, reinforced at the corners by towers which are also square; more towers rise at the centre of each side. The sturdy walls, 2.5m thick, are in alberese stone and are crowned by even "swallowtail" battlements.
Of particular interest is the entrance portal, a beautiful example of the composite culture of the period of Frederick II, with references to the Tuscan Romanesque-Gothic period in the typical two-colour covering, to the Gothic style in the capitals surmounted by lions, and to classical culture in the two half-pillars supporting the tympanum. In recent decades, after centuries in which the Castle was used in different ways, the Municipality has undertaken restoration work that has recovered the original image and appearance of the building, and made it accessible to the public. Visitors can explore the interior of the Castle and reach the walkways at the top by climbing one of the two beautifully-crafted spiral staircases in alberese stone. From the walkways one can enjoy beautiful sweeping views of the city and its surrounding areas.