The castle is where world matters are discussed. The angels have seen many forms of government, but now it is holding at some form of Monarchy. There is a king, but he can't just do anything he wants; he needs some sort of approval from his advisors.
The castle is also where representatives of the demon government come to meet with the king. Denizens of the Atrium sometimes come here, but their meetings are secret, and only the king and his most trusted advisors are "kept in the loop".
