Saturday, April 26, 2014

Denmark's Viking Fortresses

by Patricia Bracewell

They form a long, L-shaped line on the map of Denmark: Aggersborg, Fyrkat, Nonnebakken, Trelleborg. Fly to these places on Google Earth and at three of them you will see the outline of a perfect circle in the turf. Within each ring are the footprints of buildings, ramparts, gates, moats, and, more clearly, two arrow-straight roads that cross exactly in the center of each circle. These are the sites of Denmark’s ring fortresses constructed in the late 10th century. The fourth fortress, Nonnebakken, is invisible, buried beneath modern streets and buildings. 

Trelleborg. Photo Credit: Wikimedia Commons

The fortresses are not precisely the same – Aggersborg is almost twice the size of the others; at Trelleborg there are additional longhouses outside the ring and an outer defensive rampart beyond. But they are so similar that scholars believe their design must have been the product of a single mind.

Fyrkat Vikingeborgen. Photo Credit: Torben Christensen.

Whoever conceived them had some knowledge of Roman engineering: the Roman foot was used to lay out the perfectly geometrical designs, and the v-shape bases of the moats, too, follow Roman practice. Some historians suggest Slav influence in the circular shapes with their interior squares; others suspect they draw on Ottonian or Carolingian inspiration. Whatever inspired the design, these fortresses were singular. There was nothing else quite like them in the early medieval world.

Aggersborg. Photo Credit: Wikimedia Commons

In recent years archaeologists have learned a great deal about Denmark’s ring forts. Dendrochronological evidence indicates that the trees used in building the fortress at Trelleborg were felled between September of 980 and May of 981. Thus we have a general date for their construction and the name of the king who ordered them into existence: Harald Gormsson (Bluetooth).

Finds at Aggersborg indicate that some of the wooden buildings were workshops where gold, silver and iron were worked. Graveyards outside the ramparts of Trelleborg and Fyrkat reveal that men, women and children were buried there and presumably lived within the fortresses. The absence of any kind of repairs made at these sites – where the wooden longhouses and wood-faced ramparts surely fell into disrepair in a matter of years – indicates that these fortresses were abandoned within a generation of their construction.

Reconstructed Trelleborg Longhouse. Photo Credit: Wikimedia Commons

And that is the puzzle that archaeologists have not yet solved. What was the purpose of these fortresses, and why were they deserted so quickly? A massive amount of time, manpower and money went into their construction. The largest of them, Aggersborg, covered more area than the Tower of London. It’s been estimated that 5000 large oak trees would have been felled for its construction and it might have housed from 4,000 to as many as 9,000 inhabitants. Every longhouse at each fortress could have provided barracks for a warship’s crew – anywhere from 30 to 100 men. All told, they would have provided their king with a formidable army.

Reconstructed Fyrkat longhouse, interior. Photo Credit: Wikimedia Commons

The 135 graves at Trelleborg and the 30 graves at Fyrkat add to the puzzle. The presence of grave goods suggests pagan, not Christian, burial in a reign where the king boasted of bringing Christianity to Denmark. There were two mass graves – perhaps the graves of enemies from some conflict? Another grave, that of a woman, yielded pearls, and a man’s grave a silvered axe – both finds suggesting people of high status.

None of this, though, tells us what these people were doing in these fortresses, how long they were there, or the specific purpose of each vikingeborgen.

The original theory was that they were built by King Swein Forkbeard around the year 1000 to train for assaults he led on England. Scholars were forced to reconsider that theory when the tree ring evidence pointed to an earlier date for their construction. It was not Swein who ordered their construction, but his father Harald.

So, why did Harald build them? Were they for the purpose of defense against a foreign enemy?  During his reign he fortified several important trading centers, built roads and bridges, and re-fortified a wall across Denmark’s border with Germany. Were the ring forts part of that effort to fend off German aggression? That’s one theory. But if so, why was the largest fortress, Aggersborg, built on the northern tip of Jutland, far from any German threat?

Layout & location of Aggersborg. Photo Credit: Wikimedia Commons

Were they military posts, then, meant to defend against an internal enemy? Harald was only the second king of the Jelling line, and he was the first to claim kingship over all of what was then considered Denmark. His massive building efforts across the country would have placed heavy demands on the Danish population. Were these fortresses, perhaps, garrisoned with his warriors and their support staff in order to exercise tighter control over a people who resented and resisted Harald’s rule – similar to what William of Normandy would do in England after 1066? If so, it didn’t work. A rebellion led by Harald’s son, Swein, overthrew the king in 987 when these fortresses were still new. Harold’s death may explain why the fortresses were abandoned so quickly.

Or were they, in fact, so quickly and completely abandoned? I can’t help wondering if Swein might have put them to some use in mobilizing his armies against the English, even if he didn’t build them. My historical sources are silent on that topic, so take that suggestion with a grain of salt. It is my own theory, and no one else’s. 

And so the puzzle remains unsolved. At Fyrkat the south-west quadrant has been left unexcavated, awaiting future study. A renewed interest in all things Viking may lead to answers to the question of why, exactly, these fortresses were built, how they were utilized and for how long. In the meantime, Denmark’s Viking Fortresses remain a tantalizing and impressive mystery.

Photo Credit:

Sawyer, Peter. The Oxford Illustrated History of the Vikings. Oxford University Press. 2001.
Roesdahl, Else. The Vikings. Penguin Books. 1998
Griffith, Paddy. The Viking Art of War. Casemate. 2009

Patricia Bracewell’s novel, Shadow on the Crown, is the first book of a trilogy about the 11th century queen of England, Emma of Normandy. The book is available in the United States, Canada, Britain and the Commonwealth; it is available as well as an e-book and an audiobook. For more information, please visit her website and look for her on Facebook at PatriciaBracewell/Author.


  1. Thanks for taking the time to write this. Extremely interesting, and the pictures are fascinating.

  2. So nice to revisit Denmark again! Do get the catalogue of the current British Museum exhibition, "Vikings: Life and Legend" as it address the purpose and fate of Swein's fortresses. New scholarship is arising all the time - happily for us.

    1. Thank you for that suggestion and the information, Octavia. That is marvelous news. I'm ordering that catalogue right now!

    2. Could you give us an update, Octavia? :)

    3. Thanks for the nudge, Octavia. Mine is now ordered. I had hoped to go, but... However, I've just realised that I missed the cinema viewing on 24th! There's another in early June.

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