The Round Tower, formerly Stellaburgis Hafniens, is a 17th-century tower located in central Copenhagen, Denmark, and one of the many architectural projects of Christian IV of Denmark, built as an astronomical observatory. It is most noted for its equestrian staircase, a 7.5-turn helical corridor leading to the platform at the top at 34.8 meters above ground, and for the expansive views which it affords over Copenhagen. The tower is part of the Trinitatis Complex which also provided the scholars of the time with a university chapel, the Trinitatis Church, and an academic library which were the first facilities of the Copenhagen University Library which had been founded in 1482. Today, the Round Tower serves as an observation tower for views of Copenhagen, a public astronomical observatory, and a historical monument. The Library Hall above the church is only accessible along the tower’s ramp and is a venue for exhibitions and concerts.

More details: https://en.wikipedia.org/wiki/Rundetaarn