The Christiansborg Palace is the largest building on Slotsholmen and holds the Danish Parliament, Supreme Court, Prime Minister's Office, and royal reception rooms. The current building only dates to 1928, but it's the sixth structure to stand in this spot, and the location has been central to the city's history for over 800 years. The most interesting sections are the castle ruins - an exhibit on the remains of a 12th-century fortress - and the docent-led English-language tours of Queen Margrethe's royal rooms. Admission varies depending on which section of the complex guests choose to visit, but most are around 50 DKK.