Christiansborg Palace on Slotsholmen in central Copenhagen is home to Denmark's parliament, the Folketing, the Supreme Court. Furthermore, the royal family is still using several parts of the Palace, which served as the main residence of the kings in Copenhagen until the 1794.
The castle was burned and rebuilt twice, in today's Christiansborg are still parts of the three performances of the castle. The buildings around the arena (stables, riding house and the Theatre Museum in Court Theatre) is from the first Christiansborg, as does the Thorvaldsen Museum, originally wagon yard, but which now appear very different. From the second Christiansborg Palace Church, the only entire building that is left. However, there is a portion details and parts which have been recycled in different places.
More info: http://www.ft.dk/Folketinget/Christiansborg_slot.aspx