The fortification tower served as a city gate at the end of the Steinweg for traffic heading towards Frankfurt. In 1709, Landgrave Karl had an observatory installed at the top of the tower with state-of-the-art astronomical instruments for observing the heavens in a rotating dome. Famous scientists from the Landgrave's court, such as Tycho de Brahe, Jost Bürgi - who set up the logarithm tables here in Kassel - or Denis Papin also worked here.
For a time, the lower part of the tower served as a prison. In 1779 it was fundamentally rebuilt and connected to the Museum Fridericianum. On the tower is a coat of arms stone from 1554 and an interesting sundial from the 18th century. It is painted and has Roman hour numerals and curves of constellations and months.
