Viking Forts: Complexes of Viking Defensive Structure (Part 1)

Posted by Ms Elly on

Viking Forts: Complexes of Viking Defensive Structure (Part 1)

Long gone the glory of the Vikings but their legacy lives on. However, what we often see about the Vikings on the media was not the complete image of the real Viking Age. No one denies the fact that the Vikings raided and looted, from hit-and-run attack to territorial conquest. But merely savage warriors who knew nothing but killing could not go that far. Besides being shipbuilding master at the time, the Vikings were excellent at building defensive Viking forts which are known as the Viking trelleborg.

Before kicking off, we should learn what type of fort did the Vikings use.

It was the Viking Trelleborg. It was the ring-shaped fort or a circular fort that the Vikings erected, commonly in Sweden and Denmark. The Vikings didn’t invent this type of fort. Rather, they learnt it from people they raided. The circular fort from other regions, according to the archaeologists, was not as strict and geometrically precise as the Viking ones.

Viking trelleborg is Viking defensive structure. Though the Vikings copied this structure, the archaeologists believed the Viking forts were the most strictest and most geometrical precise.

Viking trelleborg is Viking defensive structure. Though the Vikings copied this structure, the archaeologists believed the Viking forts were the strictest and most geometrical precise. 

During the reign of King Harald Bluetooth, the Vikings lost control of Danevirke and southern parts of Jutland to the Saxons. The complexes of Viking trelleborg in Denmark are allegedly work of King Harald Bluetooth. Usually, the inhabitants abandoned the trelleborg within a generation. Nonetheless, the purpose of the Viking trelleborg has been a matter of debate for the scholars. Some firmly believe that the Vikings (probably people of King Harald Bluetooth) built the fort for the defensive reason while others regard the complexes as the means to show off wealth and power.


Aggersborg is the largest Viking trelleborg that the archaeologists have found in Denmark. Resting in Aggersund, Aggersborg could date back to 980A.D.  The fort measures 240m in diameter. Four main roads crossed one another in the trelleborg. Surrounding the fort is the circular wall that is supposed to protect the inhabitants inside.

Viking Aggersborg picture from above. This site is allegedly a work by Harald Bluetooth

The Aggersborg could house up to 50 longhouses

Viking Aggersborg could house roughly 50 longhouses inside

The archaeologists believed that the Aggersborg could house 5,000 men and about 50 longhouses.


Vallø Borgring or the Borrering is another Danish trelleborg dating back to the 10th century. The fort is allegedly the work of Harald Bluetooth in Zealand, Danmark. The diameter of the fort reaches 145 meters and it is protected by 10-meter ramparts.

Viking Borgring detected by LIDAR

Borgring fort dating back to the Viking Age detected by LIDAR technology

Finding this Borrering was a bit of sheer luck for the archaeologists. It was not until they used LIDAR that the Borrering appeared. According to the archaeologists, even though they stepped on the site, there was nothing that could convince them that the Vikings once erected a fort there. Because the site became a favourite agricultural site meaning the peasants would cultivate it levelling the fort.

Borgeby Castle

Borgeby Castle laid its foundation on a Viking trelleborg. It is located in Scania, southern Sweden and dated back to the 11th century.

Borgeby Castle in the site of the Viking fort Viking trelleborg


Borgeby castle from Google map

The fort measures 150 meters in diameter. The archaeologists believe this too was the work under the order of King Harald Bluetooth.

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