In 1806, during the Napoleonic wars, Kassel was occupied by French troops.  The next year Kassel became the capital of a Napoleonic puppet kingdom called the Kingdom of Westphalia.  It consisted of the Electorate of Hesse (Hessen), the Duchy of Brunswick (Braunschweig), and smaller pots of Hanover and Prussia.  Napoleon's youngest brother, Jérome Bonaparte,  lived in Wilhelmshöhe Palace as the king of this  puppet kingdom from 1807 to1811.  He altered the interiors of the palace to suit his own taste, and it was under his reign that the three separate wings of the castle were connected.  During the period of the Kingdom of Westphalia, Wilhelmshöhe was given the new name Napoleonshöhe,   The Kingdom of Westphalia lasted until 1813 when, after the Battle of Nations (Völkerschlacht) at Leipzig, the kingdom disintegrated and the old German states were reestablished.

See our article about Napoleon's Great Art Robbery from Kassel too

Together with Wilhelmsthal Castle outside of Kassel -- it  is now the only castle in Hesse (Hessen) to display an interior so finely furnished in the early neo-classical and Empire styles of the period around 1810.  Neo-classical wall and ceiling stucco, Louis XVIIth and Empire style furniture, and marble sculptures based on ancient originals, vividly illustrate the changing taste of the aristocratic class as it shifted away from the baroque.  The noble and elevated simplicity of the domestic rooms testifies to the more private character of stately homes in the late 1700's and early 1800's as the rigid courtly ceremonies of absolutist rulers, designed to demonstrate their power, began to wane.  Later in 1871, Emperor Napoleon III of France, a prisoner after the Battle of Sedan, was held as prisoner in the palace.

During World War II the central part of Wilhelmshöhe are castle was severely damaged.  After the war the castle was rebuilt, however without the reconstruction of the cupola in the middle.  And the conscious decision was made to replace the windows in the central building without mullions, so that the palace retained a visual reference to a building without windows, one that has been destroyed and had the windows blown out by an explosion.

The dining room in the public reception rooms is notably faced with painted stucco rather than silk wallcoverings.  Precious textiles were the most expensive item used for furnishing in those days, and here the silks would have been exposed to the smells of the food.  The banquet table, now laid out in festive manner, used to be put out only on special occasions and packed away again after the meal was over.  The table arrangements at Wilhelmshöhe boasts a particularly magnificent centerpiece by Pierre-Philippe Thomire, a bronzer in Paris under Napoleon Bonaparte.  The inset mirrors reflected the light of the two big crystal chandeliers, casting a ceremonial radiance over the table settings.

In the 1820's the original terraces that connected the three wings of Wilhelmshöhe Palace were replaced be connecting rooms on each of the three storeys.  Today the connecting rooms between the Weissenstein Wing and the main building where the art museum now is, still have the old wall fittings with original Berlin stoves in niches adorned with stucco.  After the main building of Wilhelmshöhe Palace and the winter residential palace in the city were both destroyed in World War II these connecting rooms were used to display the precious items of furniture that had been rescued from the firebombing.  This includes the Elector Wilhelm II's bedroom with the imperial bed of 1823.  With its lavishly gilt bronze ornamentation and monumental fourposter, this room was more than a place to sleep.  As part of the elector's a ceremonial suite it played its part in demonstrating the prestige of the regional aristocracy.

The bathroom, from 1825, had walls painted to imitate the recently uncovered murals in the Roman villas of Pompeii.  The black marble tub is recessed into the floor.