Slotsholmen is the seat of national government and a veritable repository of historical sites. Located on a small island and separated from the city centre by a moat-like canal, Slotsholmen's centrepiece is Christiansborg Slot, a large palace that is home to Folketinget (the Danish parliament) and various government offices.
The original Christiansborg palace was constructed in the 1730s to replace the pokey Copenhagen Castle and several buildings, namely the royal stables and edifices surrounding the main courtyard, date from this time.
Folketinget, the parliamentary chamber, can be toured on Sunday year-round, as well as on weekdays over summer, and this includes a peek at Wanderer's Hall, which contains the original copy of Denmark's Constitution.
For sheer Renaissance grandness, De Kongelige Repræsentationslokaler (the Royal Reception Chambers) won't disappoint - it's where royal banquets are scoffed and heads of state entertained. Underneath the palace are the excavated ruins of two earlier castles, including Bishop Absalom's original 1167 effort.