The Ottoneum, built between 1603 and 1605 by Landgrave Moritz the Learned, was one of the first purpose-built theatres in Germany. After the theatre moved out it became an anatomical institute, a library, and is now Kassel's natural history museum.
The collection includes an impressive set of 18th-century botanical models in wood, a large mineralogy collection, and several walk-through dioramas of regional habitats. The building's age makes it the most atmospheric museum space in the city; the renovations have left timber beams, brick walls, and unexpected staircases on display. A good rainy-day option for anyone travelling with children.