Construction, widening and reconstruction of the stronghold went on throughout several centuries, with the architecture changing according to the development of weapons. The stronghold achieved its final dimensions – area of more than 30,000 square metres, thickness of the wall between , and maximum height over – under the reign of Bishop Johannes IV Kievel (1515–1527). The western side of the castle houses a watchtower dating from the 13th century, later used as a bell tower. The walls were later raised to .