The Round Tower (Rundetaarn) is the 17th-century brick tower on Købmagergade in central Copenhagen, built by King Christian IV as an astronomical observatory. Instead of stairs, a wide spiral ramp winds up inside it, so smooth that in 1716 Tsar Peter the Great rode his horse all the way to the top. It is still home to Europe's oldest functioning observatory.
The Round Tower at a glance
- Where: Købmagergade in Copenhagen's old town, attached to Trinitatis Church, a few minutes' walk from the Strøget shopping street.
- Built: 1637–1642, for King Christian IV.
- By: the design is usually attributed to the royal architect Hans van Steenwinckel the Younger.
- What it is today: a viewing tower about 35 metres tall with an exhibition space in the old Library Hall and a working public observatory at the top.
- Time needed: 45 minutes to an hour for the ramp, the Library Hall and the view.
An observatory, a library and a church in one
Christian IV built the Round Tower as part of the Trinitatis Complex, which combined three things the university needed: an observatory, a library and a church for the students. The tower was finished in 1642. The ramp, about 209 metres long and winding seven and a half times around the hollow core, was designed so that heavy instruments, and books for the library, could be brought up by horse and cart.
The university library filled the loft above the church, reached from halfway up the ramp, and lost much of its collection in the great fire of Copenhagen in 1728. It remained the university's library until 1861. The observatory was used by university astronomers until the same period; the current telescope dates from 1929, and the observatory is often described as the oldest still working in Europe. Today the old Library Hall hosts exhibitions and concerts.
What to look for
- The gilded rebus on the façade, designed by Christian IV himself. With a heart, a sword and Hebrew letters for God, it is usually read as a prayer: 'Lead, God, the right doctrine and justice into the crowned Christian IV's heart, 1642'.
- The spiral ramp: a cobbled path with no steps, with small windows along the outer wall and doors into side rooms.
- The glass floor about 25 metres up: stand over the tower's hollow core and look straight down.
- The Library Hall halfway up, a long vaulted room above Trinitatis Church.
- The wrought-iron railing on the top platform, with Christian IV's monogram and his motto 'Regna Firmat Pietas' (piety strengthens the realms), and the view over the old town's copper spires and roofs.
The Round Tower in a fairy tale
Hans Christian Andersen used the tower as a measure of size. In his fairy tale The Tinderbox, the soldier meets three dogs guarding treasure, and the last of them has eyes as big as the Round Tower. Danes still use the tower as a yardstick for anything enormous.
Tips for your visit
- The ramp is gentle but long: comfortable shoes help, and it is easy for children. Only the last few metres to the platform are a short staircase.
- Go up late in the day for soft light over the roofs. In the darker months the observatory sometimes opens on evenings for stargazing; check before you go.
- Combine the tower with a walk along Købmagergade and Strøget, or with Rosenborg Castle, another building of Christian IV.
Nearby
- Trinitatis Church: the university church built together with the tower, which you can visit from the street side.
- Copenhagen Cathedral (Vor Frue Kirke): a short walk away, with Bertel Thorvaldsen's marble statues of Christ and the apostles.
- Rosenborg Castle: Christian IV's Renaissance castle in the King's Garden, home to the Danish crown jewels, a short walk north.
- Strøget: the long pedestrian shopping street between City Hall Square and Kongens Nytorv, just round the corner.
See Copenhagen with World City Trail
Our Copenhagen scavenger hunt turns the old town into a game you play on your phone. The route links 10 stops and takes around 3 hours, with about 48 minutes of walking. The Round Tower is one of the stops, along with City Hall and Copenhagen Cathedral.
If you prefer a different format, there is also a self-guided walking tour and an outdoor escape game in Copenhagen.
Play the Copenhagen scavenger hunt →
Take the self-guided walking tour of Copenhagen →
Play the outdoor escape game in Copenhagen →
Facts checked in October 2026.