Sometimes referred to as "the City of Spires", Copenhagen is known for its horizontal skyline, broken only by the spires and towers of its churches and castles. Most characteristic of all is the Baroque spire of the Church of Our Saviour with its narrowing external spiral stairway that visitors can climb to the top. Other important spires are those of Christiansborg Palace, the City Hall and the former Church of St. Nikolaj that now houses a modern art venue. Not quite so high are the Renaissance spires of Rosenborg Castle and the "dragon spire" of Christian IV's former stock exchange, so named because it resembles the intertwined tails of four dragons.