When the Emperor Frederick II erected this castle close to Bari in the Puglia distict of Italy in the 13th century, he imbued it with symbolic significance, as reflected in the location, the mathematical and astronomical precision of the layout and the perfectly regular shape. A unique piece of medieval military architecture, Castel del Monte is a successful blend of elements from classical antiquity, the Islamic Orient and north European Cistercian Gothic.
Frederick II was a scholar and an architect. He knew Arabic as well as Latin, and he was influenced by all he'd seen on a crusade into the Arabic world. He founded the University of Naples. He was also a friend to the great mathematician Leonardo Fibonacci, who introduced the Arabic representations for zero and ten into European calculation. Frederick's architecture is marked by strong planar surfaces with circular towers and half-towers. It's all very geometric. In the words of one architectural historian, it makes no concession to the land around it. And the imprint of abstract Islamic ornamentation runs through Frederick's work.
He built the Castel Del Monte in the 1240s. It's a perfect octagon, 123 feet across, with an octagonal center court. Octagonal towers on each corner carry the octagonal theme downward in scale. Its floor plan reflects octagonal Islamic tile work. It's an eye-catching exercise in geometry. Castel del Monte is a landmark, and stands on the brow of a very high hill, - the extremity of a ridge that branches out from the Apennine. The ascent to it is near half a mile long, and very steep; the view from its terrace most extensive. A vast reach of sea and plain on one side, and mountains on the other; not a city in the province but is distinguishable; yet the barrenness of the foreground takes off a great deal of the beauty of the picture.
The building is octangular, in a plain solid style; the walls are raised with reddish and white stones, ten feet six inches thick; the great gate is of marble, cut into very intricate ornaments, after the manner of the Arabians; on the balustrade of the steps lie two enormous lions of marble, their bushy manes nicely, though barbarously, expressed; the court, which is in the centre of the edifice, contains an octangular marble bason of a surprising diameter. To carry it to the summit of such a hill must have cost an infinite deal of labour. Two hundred steps lead up to the top of the castle, which consists of two stories. In each of them are fifteen saloons of great dimensions, cased throughout with various and valuable marbles; the ceilings are supported by triple clustered columns of a single block of white marble, the capitals extremely simple.
The interior of this ancient building is also extremely striking; the inner court-yard and great Gothic Hall, invested with the sombre mystery of partial decay, the eight rooms above, the numerous windows, all would repay a long visit from any one to whom the details of such architecture are desiderata. So why did Frederick build it? His castles all over Sicily and southern Italy were certainly meant to express imperial power. But in this building we see something more. It's a dark and closed place inside, with a form so coolly mathematical as to be beautiful. A contemporary looked at it and wrote Stupor mundi et immutator mirabilis. That was more an astonished outcry than a sentence. It meant something like, Amazed world -- wondrous novelty!