The fortress is an octagonal prism with an octagonal bastion at each corner. Each wall has 8 rooms and an eight-sided courtyard occupies its centre. The octagon is thought to be 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 whilst on Crusade, or by the Palace Chapel of Aachen Cathedral.
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 due east.