Even though Fredrick II did not visit Castel Del Monte very much they still had pretty good defense. They had people in the watch towers so if people tried to scale the wall or use a battering ram the people in the watch towers could shoot them with their weapons. There is not a moat in this castle. Another defense system is the location of doors to make it harder to get from room to court yard to room or room to room. This castle looks strong.
Reference: Gotze, Heinz. Castel Del Monte. New York, Prestel, 1998.