The 17th century tower and observatory Rundetaarn, or the round tower, is the oldest functioning observatory in Europe.
When Christian IV built the tower, Denmark was quite famous for its astronomical achievements thanks to the astronomer Tycho Brahe. When he died in 1601, the King wished to continue Brahe's research, and thus the round tower came into being.
It has been a while since the scientists left, but the observatory is still used by amateur astronomers and the many visitors. The observatory is encircled by an outdoor platform from which you have a magnificent view of the old part of Copenhagen.
To get there you need to walk up the spiral walk, which is 268,5 meters long at the outer wall and only 85,5 meters long close to the core of the building. This means that you walk around 209 meters to get to top even though the tower is only 36 meters tall.