Christiansborg Palace on Slotsholmen island is the seat of the Danish Parliament and the home of Danish political power. It is also open to visitors — unusually for a functioning government building — with tours of the Royal Reception Rooms, the Parliament, the ruins of earlier castles beneath the floor, and the royal stables.
The current building, finished in 1928, is the third palace on the site. It is an imposing granite neoclassical structure with a tower that you can climb for free views over the city. The banquet hall contains a series of large tapestries commissioned in 1990 depicting Danish history from the Vikings to the present. The underground ruins museum shows the remains of Bishop Absalon's original castle from the 12th century — the earliest fortification on what became Copenhagen.