A Viking Network Info-sheet:

The viking fortresses of Fyrkat and Aggersborg

by Claus Hagerup Jensen, Soeren Christensen, Brian Skov, Marianne Hejlesen, Louise Andersen


Noerresundby Gymnasium og HF-Kursus

Feruary 1996

Denmark at the time of the building of Fyrkat and Aggersborg

In Denmark there are four viking fortresses and in Sweden there is one, which can be counted to the Danish ones. These five castles are : Fyrkat, which lies in Himmerland near the city of Hobro. Aggersborg, which lies by The Limfjord near the town of Aggersund. Nonnebakken, which no longer exists, but is located in the city of Odense. Trelleborg, which lies near the city of Slagelse on the western part of Sealand. The swedish one is also named Trelleborg, in Skaane, close to the city of Trelleborg.

Denmark was in a difficult situation, when the central power in Denmark decided to build the five known "Trellecastles" on the Danish territories of that time. It is very possible that it was a combination of several causes, that made the rulers make this decision. Earlyer, the following mentioned were dominating the discussion:

  1. Fear of the neighbours to the south, which the rampart Dannevirke is a proof of actually existed. They could have wished to have fortified military installations in the north, if the Germans were to cross the borders at Dannevirke.
  2. The vikings had a wish of expansion, firstly towards England, and terefore the king needed some training camps for his soldiers.

The dominating theory today is, that the castles were to be used by the Danish King to suppres the local chiefs and their small "kingdoms".

This is documented in the writing on the great Jellingstone : Quote from the paper produced by the group working with Jelling.

It is quite obvious, that there were many good reasons to build the castles these places, and there is no doubt that the local chiefs could be persuaded to accept the central regime. And there does not appeare to have been any great conflicts about the throne of the realm.

We do not know much about the political conditions previous to the building of the castles, and it is therefor difficult to prove the domestic perspectiv in them. But it was a quite good way to enforce the powers of the sovereign. But it is clear that the castles were not build with the sole intention of defending the lands of the crown. They were local compulsion-castles. It is fair to say this with their locations in mind. The king could also have had another motive: The domestic trade. The castles quickly became regional powercenters in the business life, and that gave the king an opportunity of controlling the trade.

All in all it is fair to conclude, that many factors probably lies behind the building of these "Trellecastles", but that the dominating explanation must be the wish of the monark to subdue the whole kingdom. And that this monark must have had support from a great and strong army, which also benefited from these castles.

All of the fortresses are from the period of King Harald Blaatand, son of Gorm the Old, which means: They have been built around 980. (Although the Sweedish fortress has not been precisely dated at the moment, it is sure, that this fortress is from about 1000 A. D. +/- a hundred years).

The history of Fyrkat

In 1950 the National Museum of Denmark made a smaller trial excavation of Fyrkat voldsted in Onsild Brook Valley by Hobro. That was with economical support from the council in Hobro. The excavation was made due to theories, that there might be a round viking castle something like Trelleborg (excavated just before the second world war) by Slagelse. The architecture itself at these castles is very fantastic, but we will tell a little about that later. From the air you could faintly see something, and you could also see on the crop that there was something in the soil which, manured the plants very much. The crop therefore either was matured before others, or was neater or higher than the other. This phenomenon is also seen at the excavations by Sebbersund, which is recently discovered. It showed up to be a round viking castle, and because of that, a great excavation of the place began the year after. It was under the leadership of the architect C. G. Schultz. The unbelievable comprehensive diggings were carried out by long-term unemployed people from Hobro and the surrounding country, who got economical support from the Working- and Social Ministry with a pile of money. The Carlsberg Foundation supported the scientist's work with many contributions, and during three seasons foreign students helped with the excavation work. The museum director died in 1958, but the excavation was almost finished then. The course of the bank and the grave were investigated completely, and 10 of the 16 longhouses had been marked with white concrete pillars, to show how and where the bearing beams stood (take a look at the introductionsheet to this project, and you will see those longhouses). Left was only the castle's south-western quarter. It still lies there, unexcavated, because future archaeologists ought to have the chance to check up on the results of the research. Already very early in the research process there was a lot interest about the excavation. People would like that Fyrkat should be a historical monument. In 1953, the National Museum of Denmark started a collection to buy Fyrkat Millfarm. That was to avoid that the farm would become dilapidated, if it was owned by a private person. They could already close the bargain the year after, in 1954. Those moats, which was filled up with soil was emptied, and the large earthwork which surrounded the castle was reestablished. Inside the castle itself was the cross-formed road network and the stake holes was pointed out by white concrete pillars. By all those arrangements the National Museum of Denmark has among other things tried to show the Viking castle's special structure and character. Since 1969, Hobro Museum has exhibited the finds from the castle. They have even arranged a whole room with these finds from Fyrkat. It must be pointed out, that Hobro Museum is a small, but beautiful half-timbered house with two floors. And the Fyrkat Exhibition you can find in a narrow chamber with the size of 2 x 2 meters. Although it is a great experience to see the finds from then.

Fyrkat at the Viking Age

Fyrkat was built in the years 980 A.D. to 981 A.D. by King Harald Blaatand. At that time, a sound was making an inlet from The Firth of Mariager right beside the west side of the castle. Therefore, it was a very swampy area, which the castle was built on. The valley, where Fyrkat was lying, was a very flat one. The vikings could see far away and discover enemies very quickly. Around the castle ground, there was many small brooks. They formed good sailing routes, but they could not be used as an attack road. They were simply too narrowed. Where the castle didn't bordered on to the sea, it was surrounded by a great moat. There are many different theories about, what the old viking castle has been used to. Earlyer, many believed is has been a training place for marine infanterists. In the viking age, Denmark looked a lot different from what it looks like today. The first theory is very unlikely, because there has been found women's ornaments and women's tools. That is a/o. about large looms, wool- or flax combs and spinning wheels. There is not found concrete finds of the first- and the last-mentioned, but there is found spinning wheel- and spindle weights. This observation results also in the fact, that this fortress has not simlpy been a military basecamp (although it certainly has been a central fortress of King Harald, built against some local chiefs). The women lived together with their husbands at the fortress. Among the finds we can mention: Pearls made of glass, silver, karneol, oxydian and amber. There is found bracelets of silver and a piece of a neat made gold granulated ornament with filigree work. Toe rings and finger rings of silver has also been found. The archaeologists also excavated bronze chains and a bronze button, which has made out an oval women's ornament. From these finds, which is not known from Scandinavia, one could also mention some other toe rings, and something which looks like a stick knob. The theories says, that they come from Eastern Europe. By studying other finds from Fyrkat, a/o. the adhesive stonepots, ,which comes from Norway, you can easily conclude, that the Danish society in the viking age was highly developed, regarding business economy.

Facts about Aggersborg

Aggersborg is, with its 248 m in diameter, the biggest of the "Trellecastles". It contained 48 houses which measures 32 m in length each house. This huge fortress almost covered the whole area between the church of Aggersborg and Aggersborggaard. It is so, that a part of the buildings of that big farmhouse are placed in the southern territory of the castle. The history of Aggerborggaard traces back to the year 700. From year 700 and up to 980 there was, on this locality where Aggersborg later were established, a built-up area, or to be more explicit, a village.

The fysical structure of the "Trellecastles" was a circular rampart with gates in the four points of the compass, straighten streets which divides the place of the castle into four equal parts. In these four parts there were houses which were placed as 4 winged yards.

In Aggersborg, which were the biggest of all the castles, there were 4 parts which each contained 3 blocks, wich these 4 winged yards were called.

Below: Airview upon Aggersborg.

Source: E. Roesdahl, Aggersborg.

The population on the Trellecastles

We do not know for sure, how the conditions were for the Danish population in the 10th century. There is evidence to indicate, that the areas around Aggersborg have been populated. The building of the castle must have been a great job, and was probably done by locals. Alternatively, a great deal of slaves could have been used for the job. Not only "ordinary" people lived at Aggersborg castle. Also living on the castle were taxators and other officials.

The women were in charge of the domestic. They made sure that there was a production of things woved on a loom, that all were feed, and that the house was nice and tidy.

The men were in charge of all the work there was to be done outside the house. They hunted, and took care of repairs of for instance the longhouses. In these houses lived about 20 to 30 adults, so there was not much privacy between married couples.

Despite that they were succesfull in sustaining the rate of reproduction.

The construction of Fyrkat

Fyrkat is built up so it has 16 big houses placed in the middle of a big rampart, which was lying there to pretect the buildings from other vikings. The buildings were placed four & four in four squares, but whith guardhouses in the middle. The concretesockets were filled in the posthole, which the pales has left as mould in the ground. That is why you can see which size the buildings were inclusive the buildings position in proportion to each other (look at the common introductionsheet).The houses were built of strong lumber but the walls were isolated with liquid manure and clay. There were three rooms in the houses. A chilly room where they keept their food and similar things and a room where they had a fireplace which they used to make food. They also sleeped in there. That was the only room in the house where they had warmth. The last room was a stable, which was build in extension of the houses.


There has been found weapons, which confirms, that there has been warriors in the castles. For example, a long-horned axe has been found. Here there are also things which were used by the women: Jewels, looms, spinningthings and some weapon. The axe is from about 1000 and it has been found in a grave. There has also been found many articles for everyday use, like for example journeyman tools and agricultural tools: Coulter and blades from some scythes. These things were used for the harvest in the viking age and in the earlier middle ages. There were also used a bowlshaped buckle of gilt bronze. The most magnificent jewel of the excavation is a little danglejewel of gold, with a conventionalized birdreproduction. The other things, which were found on the vikingcastle, were particularly potsherds, bones and tools but not many things were found.


Here is a few facts about Aggersborg:
bullet Aggersborgīs interior diameter = about 240 metres
bullet Biggest diameter = about 285 metres
bullet Number of houses = 48
bullet These houses lie inside the rampart
bullet The height of the rampart = 3 metres
bullet The size of the houses = 32,45 metres long, 8,3 metres wide
bullet The centre of the hall = 19,5 metres
bullet The moat outside the rampart = 1,3 metres debt, 8 metres wide
bullet The foundation of Aggersborg = around the year 1000
bulletFacts about Fyrkat:
bullet The rampartīs interior diameter = 120 metres
bullet Thickness = around 12 metres
bullet Height = 3 metres
bullet Number of houses = 16 big houses
bullet Size of the houses = about 28,5 metres long

An interpretation

The Vikings must have been very wise since they builded the rampart in 3 metres height. That served the purpose of making it easier to scout for the enemy. The history and memory of Fyrkat did unfortunately not last very long because of a big destructive fire. It is not told how Fyrkat burned to the ground and we do not know the cause of the fire, but it might have been because of a war between two viking clans. However, we do know the time of the fire. Several things that has been found around Fyrkat has been dated back to the time of Svend Tveskaeg, so it is sure, that this successor of King Harald Baatand also controlled the fortresses.

There are different opinions concerning the importance of Aggersborg. Some think Aggersborg was used in a cultic aspect or as a shipyard. But that is a matter of opinion. Yet we can toss all of these opinions away because we know now that the two castles has been dated back 10 to 15 years before the conquest of England. Therefore, we can exclude the claim of the two castles being some kind of a military base for invading England. But here is a tricky question: why was Aggersborg placed so central by the Limfjord? Well, the archaeologists have to study the area around the castle. Perhaps they will find the answer there. A good explanation is, meanwhile, that King Harald wanted to controle The Limfjord, which formed an important trading route in the ancient Danish infrastructure: A route south of the northern point of Denmark (through The Limfjord from The Northsea to Skagarack) - Skagen - which always have threathened the captaiins and their crews!


P. Hansson, Trelleborgenes ukendte fortid.

O. Olsen, Fyrkat.

L. B. Jensen, Aggersborg og omegn i vikingetiden.

H. Dehn-Nielsen, 348 oldtidsminder i Jylland.

E. Roesdahl, Aggersborg.