The building was erected as a theatre by architect Wilhelm Vernukken in 1603/1606. The commissioner was Landgraf Moritz, who named the Ottoneum after his son Otto. Today, a natural history museum is housed here. Well-known collection pieces are a Xylotheque and the Ratzenberger Herbarium. [German]
Fun detail seen in Kassel: A figure standing on top of a sphere inside a church tower. Its a sculpture created by the artist Stephan Balkenhol. The traffic lights are not art, I was just standing at a pedestrian crossing and there was no time for a better position.
After World War II, a large part of Kassel was rebuilt in the style of the 50ties. So if you are interested in the architecture of this period, a walk through Kassel is an appropriate way to learn more about it.