The Rundetaarn or The Round Tower was one of many architectural projects by Christian IV. Completed in 1642, it was built as as an astronomical observatory. It is a cylindrical tower built in masonry. Instead of stairs, a 7.5-turn spiral ramp forms the only access way to the towertop observatory as well as the Library Hall and the Bell-Ringer's Loft. The ramp turns 7.5 times around the hollow masonry core of the tower before reaching the observation deck and observatory at the top. This design was chosen to allow a horse and carriage to reach the library, moving books in and out of the library as well as transporting heavy and sensitive instruments to the observatory. The winding corridor has a length of 210 m, climbing 3.74 m per turn. Today the Round Tower serves as an observation tower for expansive views of Copenhagen, a public astronomical observatory and a historical monument. - Wikipedia