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:The Gravensteen is a castle in Ghent originating from the Middle Ages. The name means 'castle of the counts' in Dutch. Arnulf I (918–965), Count of Flanders, was the first to fortify this place, building a medieval bastion on this high sand dune, naturally protected by the river Leie and its marshy banks. This bastion consisted of a central wooden building and several surrounding buildings, also in wood.
In the early 11th century, the wooden building was replaced by a stone residence, consisting of three large halls that made up three storeys, connected by a stone stairwell. The monumental stone staircase, the light openings, the fireplaces built into the walls and the latrines were signs of considerable luxury and comfort in those days. There was probably also a tower.