The construction of the castle dates back to January 29, 1240, when Federico II Hohenstaufen ordered Riccardo da Montefuscolo, Captain of the Order, that the materials and everything needed for the construction of a castle at the church of Sancta Maria de Monte disappearance. This date, however, is not accepted by all scholars: according to some, in fact, the construction of the castle on that date had already come to the roofs. Not sure is the attribution to a precise architect: some carry the work to Riccardo da Lentini but many argue that the construction was the same as Federico I and that it was built on the ruins of a former fortress before the Norman epochian longobard. Probably at the death of Frederick II (1250), the building had not yet been completed. Castel del Monte was also a prison place. Under the jurisdiction of Charles I was imprisoned, secretly under the custody of Castello Golardo Saumeri, Corrado Conte of Caserta and the small children of Manfredi: Enrico, Federico, Enzio. The building is octagonal (outer side: 10.30 m between the tower diameters of each tower: 7.90 m) and at each edge an octagonal turret (side 2.70 m) is connected. The octagon corresponding to the inner court has sides whose size varies between 6.89 and 7.83 m. The inner courtyard diameter is 17.86 m. The diameter of the entire castle is 56 m, while the diameter of each tower is 7.90 m. The towers are 24 m high and slightly exceed the height of the inner courtyard walls (20.50 m). Although it is commonly called "castle", the exact function of the imposing building is still unknown. Architecturally devoid of typical military and ditch elements, placed in a non strategic position, the building was probably not a fortress. Some elements of the building, moreover, make a decisive discard of this hypothesis: for example, the spiral staircases in the tower are arranged counter-clockwise (unlike any other defensive construction of the time), a situation which disadvantages the occupants of the castle Against any assailants because they would have been forced to seize the weapon with the left. Even the hypothesis that it was a hunting lodge, much loved by the ruler, is questioned by the presence of fine ornaments and the absence of stables and other typical hunting lodges. Because of the strong symbolism it has embellished, it has been hypothesized that construction could be a kind of temple, or perhaps a sort of temple of knowledge, in which to devote itself undisturbed to the study of sciences. In any case it is revealed as a great architectural work, synthesis of sophisticated mathematical, geometric and astronomical knowledge. The building, besides being an example of precise construction, is loaded with symbolism that has attracted many scholars. The octagon on which the plant and its elements are based is a highly symbolic geometric form: it is the intermediate figure between the square and the circle that represents the infinity of the sky and thus the passage of one To the other. The choice of the octogenarian could come from the Rock Dome in Jerusalem, which Frederick II had seen during the sixth Crusade, or the Palatine Chapel of Aachen.