The Blauer Hut, the southwestern and probably the oldest part of the city fortification dating from 1250 to 1300, is located on the edge of the castle palace park. It owes its name to the colour of its former slate roof. The tower, which can only be accessed via the fortification, served as a prison with an 8 m deep dungeon. Part of the city wall and the former outer ward are preserved here.
The Blue Hat is a popular photo motif for locals and visitors to the city.