Rundetaarn

The Round Tower (Danish: Rundetårn) is a 17th-century tower in Copenhagen, Denmark, one of the many architectural projects of Christian IV. Built as an astronomical observatory, it is noted for its equestrian staircase, a 7.5-turn helical corridor leading to the platform at the top (34.8 meters above ground), and its views over Copenhagen. The tower is part of the Trinitatis Complex which also includes a chapel, the Trinitatis Church, and an academic library, which were the first facilities of the Copenhagen University Library founded in 1482.

Building data from Wikidata (CC0); heights normalised. Summary from Wikipedia (CC BY-SA).