The Round Tower (Rundetårn) is a 17th-century astronomical observatory built by Christian IV in 1642, rising 35 metres above the Latin Quarter of the old town. Instead of stairs, a continuous helical ramp winds 7.5 revolutions to the top — wide enough that horses could, and did, walk up it. The observatory at the top is still used for public stargazing events.
The tower is attached to Trinitatis Church and was originally part of a complex that included a university library. From the viewing platform you get one of the best ground-level panoramas in Copenhagen — rooftops, spires, and on a clear day the Swedish coast. The tower also hosts a small gallery space within the ramp itself. One of the more interesting and underrated sights in the city.