The Round Tower was built in 1642 under the orders of King Christian IV of Denmark as an astronomical observatory and part of the Trinitatis Complex (which combined a church, a library, and the observatory in a single building). The tower is 115 feet (35 meters) tall and has an observation deck at the top and a hollow core.
Visitors can ascend the tower by walking up a spiral ramp that offers views of the city from windows at different heights. Halfway up the ramp, you’ll find the entrance to the old university library, where the writer Hans Christian Andersen sought inspiration for his fairytales; it now serves as a gallery and concert venue. At the top of the tower is an observation deck with sweeping views of the old city and a glass floor that allows you to see into the building’s hollow core.