The inner harbor of Copenhagen is the channel that divides Sjælland and Amager islands. From the harbor extends a narrow arm, the Nyhavn [new harbor], lined with picturesque old houses and closed off by Kongens Nytorv, an irregular square from which the arteries of the city radiate. The Charlottenborg Palace (17th cent.) and the royal theater (opened 1874) are on Kongens Nytorv. Other landmarks include Amalienborg Square, enclosed by four 18th-century palaces, one of which has been the royal residence since 1794; the citadel (c.1662); the city hall (1894–1905); the round tower, used by the astronomer Tycho Brahe as an observatory; and the Cathedral of Our Lady (c.1209; rebuilt in the early 19th cent.), with sculptures by Albert B. Thorvaldsen. The island of Slotsholmen, with a moat on three sides and the harbor on the fourth, supports an impressive complex of buildings, notably Christiansborg Palace (18th cent.; restored 1916), erected on the site of Archbishop Absalon's original castle and now housing the Danish parliament, supreme court, and foreign office; the Thorvaldsen Museum (opened 1848); and the stock exchange (17th cent.). On Holmen island in the harbor, opposite the royal residence, is the large modern opera house (opened 2005). Favorite spots in the city include the Tivoli amusement park (opened 1843) and the waterfront Langelinie Promenade, near which is the famous statue of Hans Christian Andersen's Little Mermaid.