Frederick II built Castel del Monte in the 1240s, and a document of 1240 shows him ordering the governor of Capitanata to finish works on the castle. It was never finished. The plan is an octagonal prism with an octagonal tower at each corner and an eight-sided courtyard in the center. The main wall is 82 feet (25 m) high, and the diameter is 184 feet (56 m). The article describes classical elements, such as an entrance pediment on thin fluted pilasters. It suggests the Dome of the Rock or the Palatine Chapel in Aachen as inspirations for the octagon.
Its purpose is debated. It was once thought a hunting lodge, and scholars now believe it was a citadel. Under Charles I of Anjou it became a state prison, and the Bourbons later took its marble columns to the Royal Palace of Caserta.
The Italian State bought it in 1876 and began restoration in 1928. UNESCO listed it in 1996.

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