The south-eastern corner tower of the city fortifications was at first a powder magazine. In the 18th century, its use was changed to state prison with living space for the jailor. Delinquents were clapped in irons there, hence the name iron house. The terrible conditions there lead to multiple complaints in 1838 and in 1858/59 a new prison was erected not far from here. Since then the tower has been used as a residential building.