Round Tower

The tower was commissioned by Christian IV and is generally attributed to the architect Hans van Steenwinckel the Younger, who died in 1639 before it was done. The foundation stone was laid in 1637, and bricks were ordered from the Netherlands because local producers could not meet the quality required. Funds ran short several times, and churches across Denmark and Norway were ordered to contribute to the cost.

It forms part of the Trinitatis Complex, which also holds a church and an academic library, the first premises of the Copenhagen University Library. The church and library were completed in 1657. The complex was badly damaged in the Great Fire of 1728 and then rebuilt.

Its best-known feature is the equestrian staircase, a helical corridor of 7.5 turns that rises to the platform about 34.8 metres above ground. The first director of the observatory was Christian Longomontanus. In the early 19th century the tower became outdated for astronomy because of light pollution and traffic vibrations, and the university moved its observatory to a new site on the old ramparts.