"Castel del Monte, whose original function is still unclear, was certainly modeled on the Dome of the Rock in Jerusalem. The work was a training ground for the greatest innovator of Italian Gothic sculpture: Nicola Pisano, whom the documents describe as “De Apulia”, a native of these parts. Between 1240 and 1245 Nicola Pisano began his career under Frederick II, perhaps as an architect and engraver of cameos as well as a sculptor. He absorbed a wide range of classical culture. After further elaborating it in the light of the finest Tuscan works and renewing it by the “truth of life” from Gothic from north of the Alps, it proved sufficient to fill the art of central Italy with references to antiquity."
More on the exhibit here.