The Emperor's Castle is considered the most important Prato's testimony of 11-13th centuries architecture. It was the headquarters of the Imperial party in the Florentine countryside, seat of the Imperial Viceroy of Tuscany.
The castle's construction begun in 1237 and ended in 1248 by order of the Holy Roman Emperor Frederick II of Swabia, in the circle of his project to set under military control the principal ways of communications that brought to Germany. This strategical location made the castle an important key in the plan for the new organization of the Italian Kingdom pursued by Frederick Hohenstaufen in the years immediately after his victory at Cortenuova. This system of fortification consisting in other castles, walled villages and strongholds all in visual contact with each other.
The architect of the castle's was Riccardo da Lentini with masonry come on purpose from the south. The site of the fortress already held a small fortress, given to the Emperor from a Ghibelline family of Prato, that was incorporated in the new castle.
The Emperor's Castle has a square plan, with towers to all angles and other four, two inherited by the precedent small fortress, at the center of every side of the wall curtain. The walls and the towers are crowned with the characteristic Ghibelline battlements at 'tail of swallow'. The towers of the preexisting fortification incorporate in the construction have compromised the geometric result of the plan.
The main gate, a doorway with gothic arch, is embellished with decorative elements obtained alternating stripes of white and green marble. The sculptured lion are a representation of the Imperial power.