Copenhagen: Rundetaarn.

“The Rundetaarn, or Rundetårn (Round Tower in English), is a 17th-century tower located in central Copenhagen. One of the many architectural projects of Christian IV, it was built as an astronomical observatory. It is most noted for its equestrian staircase, a 7.5-turn helical corridor leading to the top, and for the expansive views it affords over Copenhagen. Construction started 1635, completed in 1642.”