Nyköping Castle is a medieval castle from the Birger Jarl era, partly in ruins. The castle is mostly known for the ghastly Nyköping Banquet which took place here in 1317. The construction of the castle began in the end of the 12th century, when it began as a fortification. It is thought Birger Jarl expanded the building to a larger castle. During the reign of Albert of Sweden the castle was held as a fief by the German knight Raven van Barnekow, who made important improvements on the building, and later by Bo Jonsson Grip. Further reconstructions and expansions were done during the late Middle Ages. Gustav Vasa strengthened the castle further for defensive purposes and a round gun tower from that time remains today.
The medieval castle was rebuilt in the end of the 16th century by Duke Charles (later Charles IX of Sweden) into a renaissance palace. The palace burned down with the rest of the city in 1665. It wasn't reerected; in fact some of its bricks were used in the construction of Stockholm Palace. However, parts of the castle were sound enough to be used as county residence until the 1760s.
Parts of the castle were refurbished in the 20th century. Kungstornet (the King's Tower) and Gamla residenset (the Old Residence) currently house the permanent exhibits of Sörmlands museum (the Museum of Södermanland). A restaurant is located in the banquet hall and Drottningkällaren (the Queen's Cellar).
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.