Great Viking Fortresses Built By King Harald Bluetooth

A. Sutherland  - - About 1,000 years ago, legendary King Harald Bluetooth built several impressive Viking fortresses. Today, there is not much left of these once powerful ancient buildings, but re-constructions give us a unique glimpse of what life was like inside the circular ringforts.

Harald Bluetooth was the Viking king of Denmark between 958 and 970.

Aggersborg – Denmark’s Largest Viking Fortress

Aerial view of the Viking ring fortress of Aggersborg. The similarity in design with Trelleborg near Slagelse, is clearly evident. Image credit: View author informationCC BY 2.0

King Harald was famous for uniting parts of Denmark and Norway into one nation and converting the Danes to Christianity.

The impressive remains of one of the Vikings’ great ring fortresses were originally constructed around AD 980 by King Harald Bluetooth, and the museum at Trelleborg has models, archaeological finds, and reconstructions that hekp to experience some of  Trelleborg’s history, its inhabitants and the function of the fortress in the distant past.

Aggersborg – Denmark’s Largest Viking Fortress

During the Viking Age, Trelleborg was of great military importance, which is why Viking fortresses were constructed in the region.

On the north side of the Limfjord, Vikings built Aggersborg, Denmark’s largest Viking fortress.

Aggersborg served as a starting point for many of the Vikings' expeditions to England or further north in Scandinavia.

The fortress was burned to the ground in 1086, and although Valdemar Atterdag later tried to rebuild it, it was completely destroyed in 1441.

Fyrkat Viking Fortress – Why Did The Houses Lack Fireplaces?

Near Hobro and Nonnebakken in Odense, Vikings constructed a smaller fortress known as Fyrkat. The Fyrkat ringfort consisted of 16 bow-sided long houses, and it was located so far up the fjord.

Reconstruction of a Viking house at Fyrkat

Reconstruction of one of the houses inside Fyrkat Viking fortress. Image credit: Malene Thyssen - CC BY-SA 3.0

If you didn’t know about the fort’s existence, you had to look hard to find it. Fyrkat was rather small compared to the Trelleborg Viking fortress, but it was nevertheless an important strategic outpost.

When archaeologists excavated inside the Fyrkat Viking fortress, they discovered something unusual. All houses inside the ringfort lacked fireplaces. This could only mean that people lived outside the fortress and not inside the houses as one would assume.

Trelleborg Viking Fortress Is Best Preserved

The Trelleborg Viking fortress is by far the best preserved and the only one with a fortified outer ward. It is located at Trelleborg near Slagelse on Sealand, an hour's drive south from Copenhagen.

Trelleborg Viking fortress is not mentioned in any known ancient texts, which is somewhat surprising considering its importance. Construction of this huge ringforts requires a lot of hard work, many people and was time-consuming.

See also:

Jelling Stone And Legendary Harald ‘Bluetooth’ King Of Denmark – ‘Who Made The Danes Christian’

Historians suggest the absence of written sources can be explained by the fact the fortresses were apparently only in use for a very short time, perhaps for as little as 10-15 years.

Trelleborg Viking Fortress Is Best Preserved

Aerial view of the Viking ring fortress of Trelleborg, near Slagelse, Denmark. This was the first rediscovered Viking ring fortress, and the geometry is clearly visible. Image credit: Thue C. LeibrandtCC BY-SA 3.0

Trelleborg has been dated using dendrochronological dating of timber from its moats, and the results show the fortress was built around the year 980.

King Harald Bluetooth ordered the construction of Trelleborg and the other fortresses during a time when he tried to strengthen royal power in Denmark.

On one of the most famous rune stones in Denmark, the Jelling Stone, King Harald Bluetooth proclaims that he “won for himself all of Denmark and Norway and made the Danes Christian”.

It would be logical to conclude that the fortresses were part of an attempt to subdue petty kings and local chieftains, especially given the fact that a strong, central royal power was clearly the only actor with sufficient resources to undertake such an exercise.

Recreation of the fortress walls at Trelleborg.

Recreation of the fortress walls at Trelleborg. Image credit: Sven RosbornCC BY-SA 4.0

There are seven known Viking ring fortresses in Denmark and Sweden, and most of them are associated with the reign of Harald Bluetooth.

These Viking fortresses are often just called trelleborgs and they all have have a strictly circular shape with roads and gates pointing in the four cardinal directions.

The name 'trelleborg' has been translated and explained as a fortress built by slaves. The Old Norse word for slave was thrall, and the word borg simply means fortress.

All these Viking fortresses are of great historical importance, and both Sweden and Denmark are applying for admission hoping these ancient places will be recognized as  UNESCO World Heritage sites.

Written by – A. Sutherland  - Senior Staff Writer

Updated on December 10, 2022

Copyright © All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of

Expand for references