Donjon de Châteaumur is a dungeon built in the 12th century. An enclosure, either made of wood or stone, may have enclosed the keep in an almost circular area. What is certain is that, during this time, the keep was designed as an independent fortress, with a door located on the second floor and protected by a drawbridge that fit into the wall and the doorframe. The interior was divided into two by a partition wall for stability, and it had four levels: a basement and three floors with wooden floors. The ground floor stored provisions, the first floor housed the lord's residence (along with the drawbridge), featuring fireplaces and latrines, and the second floor likely had a guard room with access to the battlements. This top floor was accessible from the first floor via a spiral staircase embedded in the southeast buttress.
Perhaps in the 14th century, during the Hundred Years' War, an enclosure was added. Some parts of it are still visible because the houses around the keep's square are built against it, and there remains a fortified gate for carts. Another larger enclosure surely existed; one can see evidence, for example, just above the chapel's calvary. This could have been a wall or a palisade.
References:Rosenborg Palace was built in the period 1606-34 as Christian IV’s summerhouse just outside the ramparts of Copenhagen. Christian IV was very fond of the palace and often stayed at the castle when he resided in Copenhagen, and it was here that he died in 1648. After his death, the palace passed to his son King Frederik III, who together with his queen, Sophie Amalie, carried out several types of modernisation.
The last king who used the place as a residence was Frederik IV, and around 1720, Rosenborg was abandoned in favor of Frederiksborg Palace.Through the 1700s, considerable art treasures were collected at Rosenborg Castle, among other things items from the estates of deceased royalty and from Christiansborg after the fire there in 1794.
Soon the idea of a museum arose, and that was realised in 1833, which is The Royal Danish Collection’s official year of establishment.