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The Round Tower, Copenhagen

The Round Tower, formerly Stellaburgis Hafniens, a 17th century tower located in the center of Copenhagen, Denmark, and one of many architectural projects by Christian IV of Denmark, built as an astronomical observatory. It is best known for its equestrian staircase, spiral corridor with 7.5 turns leading to the platform at the top (34.8 meters above the ground), as well as extensive views of Copenhagen.

The tower is part of the Trinitatis complex, which also provided the university chapel, Trinitatis Church and the Academic Library, which were the first premises of the Copenhagen University Library, founded in 1482, to scholars of the time. Today, the Round Tower serves as an observation tower overlooking Copenhagen, a public astronomical observatory and a historical monument. The library hall above the church is accessible only from the ramp of the tower and is a venue for exhibitions and concerts.

The importance of astronomy has increased in 17th century Europe. Countries began to compete with each other in the creation of colonies, creating the need for accurate movement across the oceans. Therefore, many national observatories were established, the first of which was founded in 1632 in Leiden in the Dutch Republic. Only five years later will appear the observatory Round Tower, first called STELLBURGI REGII HAUNIENSIS.

After Tycho Brahe fell into disgrace and left Denmark, Christian Longmontan became the new astronomer of Christian IV and the first professor of astronomy at the University of Copenhagen. In 1625 he proposed to the king to build an astronomical tower as a replacement for Brahe Stjerneborg, which was demolished by Steenwinckel.

The original proposal by Longomontaine was to build a new observatory on the top of Solbierget Hill, now known as Valby Backe. But since there were also plans to build a new student church and a library for the university, the idea arose to combine the three buildings into one large complex. 

Already in 1622, Christian IV bought the land on which it was decided to build a complex Trinitatis. His initial plans for this place are unknown, but since it was conveniently located close to Regensen hostels and the university, he was chosen for his new prestigious project.

Although there is no clear evidence, it is generally believed that Hans van Steenwinkel Jr. was authorized to design the new building, although he did not live to see the tower completed.

From November 24, 1636, stones were brought to the site of the foundation, first from the town ramparts and then from the surroundings of Roskilde. The bricks were ordered from the Netherlands, as local producers could not meet the required high quality standards. In February 1637, a contract was signed with Henrik van Dingklage in Emden for the supply of bricks for construction. The first three lots were to be delivered in May, the next three in the following month and the rest on request.

Trinitatis complex was built for construction in a crowded area of narrow streets and lanes. First it was necessary to clear the territory. On April 18, 1637, 200 people, soldiers and staff from Bremerholm began demolishing the half-timbered houses that occupied this place.

The first stone in the foundation was laid on July 7, 1637. When Hans van Stenwinkel died on August 6, 1639, Leonard Blazius was taken to Denmark from the Netherlands as a new royal builder. Unlike his predecessor, he would become simply a transitional figure in Danish architecture, dying just four years after his arrival in the country, leaving no remarkable buildings of his own design. Several times the construction work was stopped due to lack of funds. Therefore, the churches in Denmark and Norway were ordered to contribute part of their income during the years of construction. In 1642 the tower was finally completed, although the church was completed only in 1657, and the library - in 1657.


The Round Tower is a cylindrical tower built of alternating yellow and red bricks, Oldenburg color. The bricks used were made in the Netherlands and are fired thin bricks, known as clutches or rags. On the back side it is attached to the Trinitatis church, but it has never served as a church tower.

Stenwinkel - whose name is otherwise synonymous with Dutch Renaissance architecture in Denmark - has left its signature style with the Trinitatis complex. Unlike its other buildings with their lush ornaments and extravagant spires, the complex is built in a purposeful and reserved style. Hans van Stenwinkel must have had a good understanding of the situation in Holland, knowing that the style he once learned from Hendrick de Kaiser, was completely abandoned.

Architects who are now setting the agenda in the Netherlands, masters such as Jacob van Kampen (Amsterdam City Hall), Peter Post (Mauritzheis in The Hague) and Philip Wingbuns, now prefer a style characterized by sobriety and restraint. He is now known as Dutch Baroque or sometimes Dutch Classicism. His supporters often relied on theoretical works such as the work of Andrea Palladio and Jacopo Barozzi da Vignola . Stenwinkel may have visited his native Netherlands before changing his style, but it was too early for him to see any of the buildings of that period.

Instead of a staircase, a 7.5 turn spiral ramp forms the only way to access the towertop observatory, as well as the hall library and the Loft "s" bell ringer located above the church. The ramp rotates 7.5 times around the hollow stone core of the tower before reaching the observation deck and the observatory upstairs, along the way also providing access to Ringer's Library Hall and attic. This design was chosen to allow the horse and carriage to reach the library, move books to and from the library, and transport heavy and sensitive instruments to the observatory.

The winding corridor has a length of 210 m, lifting 3.74 m per turn. Along the outer wall, the corridor has a length of 257.5 m and a 10% slope, while along the inner core wall, the corridor has a length of only 85.5 m but a 33% slope. 

The observation deck is 34.8 m above the street level. Along the edge of the platform is a wrought iron grating made in 1643 by Kaspar Finke, court painter in metal. On the grid one can see the monogram of Christian IV and the letters RFP, the letters representing the motto of the king: Regna Firmat Pietas - Godliness strengthens the Kingdoms.

The observatory is a small domed building, built on the roof of the tower. The modern observatory, built in 1929, is 7 m high and 6 m in diameter. Access is provided by a narrow spiral stone staircase from the observation deck.