The octagonal plan is unusual in castle design. Historians have debated the purpose of the building and it has been suggested that it was intended as a hunting lodge. Another theory is that the octagon is an intermediate symbol between a square (representing the earth) and a circle (representing the sky). Frederick II may have been inspired to build to this shape by either the Dome of the Rock in Jerusalem, which he had seen during the Sixth Crusade, or by the Palatine Chapel of Aachen Cathedral.