The first Vasknarva order castle (Neuschloss) was founded in 1349 on the northeastern border of Old Livonia. 1427–1442 a new castle (Vastne-Narva) was built, which became the centre of the vogtei of the Livonian Order. The castle was wracked in the Livonian War. Until the Great Northern War it was a fort of great importance, commanding the mouth of the Narva River. It has been known in Russian chronicles either as Syrensk or Syrenets. According the folklore St. Olga of Pskov narrowly escaped drowning when crossing the Narva rapids at Syrenets. Nowadays only parts of 3 meters thin walls have survived, mainly on the northern side.
Reference: Wikipedia
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.