The vassal tower-stronghold of Angerja was probably built in the 14th century. The rectangular construction was made of limestone, the walls were two meters thick and it was surrounded by a moat. Some parts of the walls remain to this day and a moat is still visible. It is thought that the stronghold was destroyed during the Livonian war.
Tower-strongholds were built by vassals to protect themselves, mostly against peasant uprisings, especially after the failed St. George's Night uprising of Estonian peasants in 1343.
Reference: 7is7.com
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.