Castel del Monte, from the real to the imaginary

What was the function of Castel del Monte, or better Santa Maria del Monte, as it was more commonly called until the 15th century?
The original octagonal architectural structure and the richness of the decorative apparatus demonstrate that, in addition to being a luxurious residence and a manor equipped with defense and territorial control systems, it also had a clear symbolic function as a "seat of power".
With the advent of the new kings of the Angevin dynasty, which supplanted the Swabian one after the Battle of Benevento in 1266, Castel del Monte, thanks to its strategic geographical position, was adapted to new military needs and sometimes used as a place of detention for illustrious prisoners.
In rather recent times, the fame and uniqueness of Castel del Monte have fueled suggestive legends, as imaginative as they’re baseless, which claim it was either a cosmic clock, an astronomical observatory, a funerary mausoleum, the temple of an imperial esoteric cult or even a spa complex. Beside fantastic suggestions, which attribute varied and imaginative purposes to the structure, we have the words of Frederick II himself, who in Latin documents speaks of Castel del Monte as a "castrum" (castle), albeit one equipped with residential features on top of the representative functions of sovereign power.