The Round Tower (Rundetårn)

Europe’s oldest functioning observatory
Talk of the town: The Round Tower was a gathering spot for the city's intelligentsia during the 1600s.

Built by Christian IV in the 17th century as an astronomical observatory and defence fortification, nowadays The Rundetårn is famous as one of Copenhagen's best observation spots. From the top, looking out over the red tile roof-tops, it's easy to imagine the city's bygone days.
The climb up the 35-metre tower can be quite a challenge, as not only is there no elevator but also no stairs: as it was important to be able to haul canons up to the top, the path is constructed as a spiral ramp. This design feature has allowed people to reach the top in a variety of vehicles over the centuries - cars, bicycles, unicycles, and, in the case of Peter the Great in 1726, a horse - but today visitors are confined to their own two feet.