Set in a grove of chestnut trees in the vicinity of the northern wing of the Palace is a seemingly rather unremarkable building that will reveal itself as a Neo-Classical gem once you step through the door. The first work of Court Architect Leo Klenze (1784–1864), it was built in 1810 for King Jérôme Bonaparte (r. 1807–1813), originally as a court theatre. Once Elector Wilhelm II (r. 1821–1831) came to power, he decided that he had no use for the building. In the years 1828 to 1830 it was converted into a ballroom by Wilhelm's own architect, Johann Conrad Bromeis (1788–1855).
The unobtrusive elegance of the facade is in marked contrast to the building's magnificent interior. Today the mural paintings in the main hall are considered the most valuable and artistically significant feature. The walls are divided up into panels decorated with grotesques inspired by Ancient Roman models, the entablature's frieze features scrolling vines, urns and exotic birds, and the barrel-vaulted ceiling has a painted coffering draped with garlands of flowers. The Ballroom building features the last surviving interior décor from the time of Elector Wilhelm II at Kassel: the town palace on Friedrichsplatz and the Corps de Logis of Wilhelmshöhe Palace were both destroyed in WWII.