towers contain staircases. The castle has two entrances, an unobtrusive service entrance and an ornate main entrance. Frederick's main entrance featured elements from classical design, and may have been influenced by Frederick's interest in Greco-Roman architecture. 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. Occasionally used as a hunting lodge under Manfred of Sicily, the castle become a state prison under the latter's victor, Charles I of Anjou: here Manfred's sons Henry, Azzo and Enzo were kept as prisoner after 1266, as well as other Hohenstaufen supporters. The main wall is 25 m high and the eight bastions each 26 m. The sides of the main octagon are 16.5 m long and those of the octagonal towers each 3.1 m. The castle has a diameter of 56 m. Its main entrance faces east. Modern era In the 18th century, the castle's marbles and other ornamentation were looted. Members of the House of Bourbon took the marble columns and window frames and reused them at their palace in Caserta. What remains now includes fragments of a knight and a re-used Roman relief, while in the Provincial Gallery of Bari there are a head fragment and a cloaked, headless bust, sometimes interpreted as Frederick II. After having been abandoned for a considerable length of time, the castle was purchased in 1876 for the sum of 25,000 lire by the Italian State, which began the process of restoration in 1928. During the Allied occupation of WWII, the United States 15th Army Air Force headquartered a secret navigational aid station called Big Fence at the Castel. In the 1950s, soil around the castle was discovered to contain a bright red compound produced by a strain of the bacterium Streptomyces peucetius. Scientists named the drug daunorubicin and further development identified a related compound doxorubicin that finds use as a chemotherapeutic agent used to treat cancer. Central to the plot of Umberto Eco's novel The Name of