The Round Tower was built as an astronomical observatory within Copenhagen.
The foundation stone was laid on 7 July 1637. After many delays, construction was finally completed in 1642.
The Tower is part of the Trinitatis Complex, which consists of the Round Tower, the Trinitatis Church and an academic library. The library is built into the loft space of the church. The church was consecrated in 1656 and the library was completed in 1657.
The interior of the Tower consists of a wide, spiralling ramp of 7.5 turns. 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.
There is an observation deck located 114ft (34.8m) above street level, just below the observatory. The deck is open to the public (for a small fee) and gives a panoramic view of Copenhagen.
In 1860, the observatory was decommissioned as a university observatory but was reconstructed in 1928, allowing amateur astronomers and the general public to use the telescope. It is still in use and open to the public from mid-October to mid-March.