Copenhagen - The Rundetaarn | Round Tower
The Rundetaarn, or Round Tower, is one of the many architectural projects of Christian IV in the 17th century. It was the first part of the Trinitatis Complex, which combined church, library and observatory in a single building. The tower was the tallest building at that time so student astronomers used its observatory to study the stars and planets. The Round Tower has a 7.5-turn helical corridor leading to the top, and the former observatory is now used to allow tourists to see views over the city.