Danish Ring Fortresses




Scholars imagine that the Danish ring fortresses were close to unassailable in

their time. They are strong and strategically located, quite obviously according

to a centrally conceived plan. They are not only military strongholds but also

constitutes examples of technical craft feats. In addition to the tangible work

effort, mathematical and engineering skills and profound organizational skills

are shown. Today we know seven ring fortresses or so-called trelleborgar, five

of which are in Denmark. These are situated from the north like this:

Aggersborg on the narrow eastern part of the Limfjord and then south at Fyrkat

at the inner end of the Mariagerfjord. Then a little bigger jump past Jellinge,

the old Danish royal seat, to Nonnebakken in / beside Odense on the island of

Funen. In the middle of the south-western coast of Zealand next to Slagelse,

we find Trelleborg, which was first excavated of all the ring fortresses, begun

in 1934. On the east coast of Zealand at Køge (just south of Copenhagen) in

2014, Vallø Borgring was discovered, the fifth ring castle. This is, of course,

not yet properly investigated.

In the now Swedish part of the former Danish realm (Scania/Skåne), under the

present-day town of Trelleborg lies a ring castle, which was investigated and

partly built up in the last few years, and a little north of Lund, as late as 1998, a

second Skåne ring castle, Borgby was found near Borgby Castle. This ring

fortress seems, in contrast to all the rest, to have been used/inhabited even in

the Middle Ages. A coin weight and a coin from the 1000’s have been found

and in the beginning of the 1100’s a stone tower in the middle of it was

erected. Aggersborg has an external wall diameter of 288 m, Trelleborg in

Zealand, Vallø Borgring and Borgby about 150 m, while Trelleborg in Skåne,

Nonnebakken and Fyrkat have about 120 m.

The most remarkable thing about these ring fortresses (except for Borgby) is

perhaps their short lifetime, about ten years and this in the light of all the

horrible work effort to build them. One of the things that put archaeologists on

the tracks of Trelleborg in Zealand, is a local ”fairy tale” of a lost warrior here.

But locally the ring wall has always been visible. It was even photographed

from the air in the early 30´s and a local motorbike club wanted to make a

motocross track there; this prompted the excavations proper.

The ring fortresses, are narrowly dated, that is to say, all but the newly

discovered Vallø Borgring at Køge near the Sound have the same building

date of 980/981. For example Vallø Borgring hardly presents anything to the

naked view, it has been discovered through advanced laser measurements of

the landscape as well as traditional archaeological sampling methods.

Impressive ring fortresses in fairly good state of today are Aggersborg, Fyrkat

and Trelleborg in Zealand.


There is a different, completely contemporary, construction about 10 kms

south of Jellinge: a 760 m long bridge structure over a swampy area, which

saved a battle army two day marches in detour. This well-constructed wooden

bridge can be perceived in the same context as the ring fortresses, the more

as the bridge’s usage time must be set for a maximum of 10 years. There are

no signs of repairs to it, and wood is rapidly deteriorating in a marshy

environment, unless it is oxygen-free. The bridge structure has then simply

been left for deterioration.

What was then the Danish realm?

About the year 980 (or more generally the late 10th century) the Danish realm

had begun to strengthen again. The Northenn peoples had sailed across

Europe’s rivers and seas for over 100 years. The vast trade network had

received goods moving in different directions and caused small towns and

trading venues to grow up. Possibly a road network could also have been built.

  1. (This last is probably a little speculative. We know the bridge at Jellinge

and we know the so-called Hærveien [Army road], which is pre-Viking age and

mentioned in early sources, ie. along the eastern Jutland water-deviding line,

which offered comfortable land travel without troublesome river valleys and

swamps. Probably there were at least cleared paths here. This Hærveien will

run by Hedeby, Jellinge, Fyrkat and Aggersborg. To say anything more of road

networks is probably unsafe, although Denmark’s general geography can be

said to be well-suited for roads. The most important mode of transport was by


But politically, the realm of the Danes was severed in the early 900’s after

being stronger during the 9th century. There were local chieftains who became

powerful enough to be able to covet the rule throughout the Danish area. One

such was Gorm the old, who has been counted as ”the first king of Denmark”.

This is to say, as far as we know, that he ruled only over Jutland, probably

through a patchwork of associated local nobles. Gorm was rather ”the first

among equals” (primus inter pares). So-called kingly power was not hereditary,

but the sons of the king would then have to fight for it. Gorm, dead in 958,

according to an interpretation of the rune stones at Jellinge, is said to be

buried in one of the two Jellinge mounds and his queen Thyra in the other.

However, this is unclear.

Gorm’s son Harald Blåtand (Bluetooth) should then have been aware of his

insecure position and seems to have been aiming at eliminating rivals. The

highborns had their own armies and fleets and must be kept an eye on. When

one of Harald’s grandsons returned from a foreign journey and began to spend

a lot of time with the nobles, Harald is said to have let murder him in an

ambush. Blåtand seems to have added the Danish islands alongside Skåne to

his realm of alliances.

But even Harald’s position was threatened. South of the

border beyond Danevirke, built far earlier in the 8th century, for example,

Emperor Otto threatened him with weapons. He must have looked longingly at

the Danish riches, that is to say the fertility of the country, but for the rest of

Christianity, his statements about redeeming their souls from their pagan faith

should have seemed laudable.

But Harald went ahead of him by taking the baptism, which he recorded on the

greater of the Jelling stones: ”The Harald who won all Denmark and Norway,

and made the Danes christian”. The word ’Christian’ is best understood as the

thinnest surface veneer for now. In addition, he made an extra reinforcement

to Danevirke (according to a tradition, Gorm’s still surviving queen Thyra would

have done this). But Otto nevertheless struck through Danevirke in 974 and

occupied the trading town of Hedeby. The population, who still worshiped the

Nordic gods (according to the Arab Al-Tartushi), was probably not so fond of

the baptism of Blåtand, especially when it turned out to be in vain. (In

Denmark, pagan funerals have been found well into the 1000’s.) Harald

needed to keep his grip on his empire.

Ring Castle Strategy

If you try to imagine a ring castle, hypothetically, near Jellinge a little north of

Funen on the east coast of Jutland, you can clearly see the semi-circle of ring

fortresses through the inner parts of the Danish area. (A ring castle at Jellinge

has not yet been found, but recent excavations have unearthed a strong and

wide palissade around the Jellinge area.) We will later describe the structure of

the ring fortresses, but already we mention that within them were four long

houses in 4 squares (with a square open yard in the middle), ie. in total 16

long houses. Depending on the long house length, which in turn was due to

the inner diameter of the fortress, it has been estimated that the ring fortresses

could accommodate 500-800 warriors each, which at that time is a

considerable force. (Aggersborg would have had 48 long houses and close to

5000 warriors).

Each castle will then be at about 30-40 kilometers distance, a

normal day march in let’s call it an easily walked terrain (with or without road /

path). Vallø Borgring is located at Køge Ådal, which during the Viking era was

a sailing fjord, from where one could reach the Scania parts of the realm within

one day.

Now let’s look at the locations of the ring fortresses. They are all well away

from the outer borders of the Danish area and seem not to have guarded

against external enemies. They are close to coasts, but retreated from the sea

itself and in connection with this only through a narrow waterway. They,

therefore, are not in themselves natural harbour places and they are closed to

marine attacks through the narrowness of their watercourses and at the same

time not given starting points for marine assaults. They, however, offer

waterborne transport of food or warriors. We can take the case of Fyrkat

furthest into Mariager fjord. It was well protected by partly marshland and a

river. From the fortress through the swamp there was a buried canal to the


As for Trelleborg in Zealand, the castle is again partly surrounded by

swamp and in the confluence of two rather windy river branches, pouring 3 km

away into Stora Bält, a strategic watershed. However, the river only permits

arrival by small ships.

Aggersborg is located on the north side of the narrow eastern part of the

Limfjord, protected by a front island and at the crossing of Hærveien and

Limfjorden. This, of course, is a very strategic place, far into the country, but

this time with full access for larger ships. There is no big reason to linger at the

ring fortresses covered by contemporary city centers and their original possible

strategic conditions, such as the Nonnebacken, in principle, covered by

contemporary Odense and Skåne Trelleborg covered by the Swedish

contemporary city of the same name, in addition to considering the possibility

of that the modern cities could have originated as small commodities / trading

venues under protection of these fortresses. We still have to deal with Borgby

northwest of Lund in Skåne. Borgby is located at a small river a few kilometers

from its outlet in Öresund, the same picture as for Fyrkat and Trelleborg in


We have the currently understood narrow period for the ring-fortresses’

building, namely 980-981. Previously, the intended dating stretched further,

and thus the historical understanding was another. Harald’s successor Svein

Forkbeard conquered England in 1013 England, which he held briefly, to be

followed up by the Danish Canute the Great, whose North Sea empire lasted a

little longer. The Ring fortresses’ structure and location do not match the

purpose of establishing camps for warriors for maritime conquest purposes.

During Harald’s time there were no such war campaigns out of Danish

territory. And the building time is under Harald’s time. The purpose must have been another, most probably to keep the population and its aristocracy at bay.

Harald must have had wide views. Consider the enormous work effort behind

which there is a clear will for centralization and power. He wanted to create a

kingdom in a more modern sense with geographical boundaries. However, he

lived in a time when loyalty ties between people still were the norm and their

fluidity and nature did not allow strict geographical boundaries. The price, both

economically and in measure of confidence, for the building of these

ringfortresses may have been high and we will now discuss the structure of


The structure of the ring fortresses

The fortresses are built according to a strict common pattern. It is not true that

some people built their own kind of ring castle here and others their kind of

ring castle there, instead there is a clear management plan. The fortresses

consist of perfect circular earthen walls with 17 m width and 5 m height, (slightly

varying per ring fortress). There are 4 ports, basically straight to the

north, to the south, to the west and to the east. This applies regardless of what

is outside: a river, marshland or land. If there were solid land outside, there

were also moats outside. You can walk straight through the ring castle (in the

present time when the ports are missing) in the direction north-south and west-

east without encountering houses / remnants of houses. Probably there were

wooden streets corresponding to this.

In the midst of these 4 circle quadrants each had 4 long houses to form a

square open yard. Some extra house/s/ could also be found. Within

Aggersborg’s circular wall each quadrant accommodates 3 groups of these 4

long houses. Between the house groups and the inside of the walls there is

space for a circular road. The moats were of about the same width as the walls

and 3-4 m deep (in Aggersborg’s case 1,5 m). Aggersborg’s moat almost

circumreaches the fortress. Scholars do not think this moat was filled with

water but instead studded with pointed poles.

Trelleborg has some differences as compared to other ring fortresses. In front

of the outside part with solid ground there is an outlying wall partition

concentric with the main wall with a number of long houses with their short

sides towards the inside of this exterior. Their longitudinal axes meet in the

center of the ring fortress. We speak literally about correct geometry in all

respects. The house lengths are exactly the same for each ring castle,

somewhat varying per castle and in Trelleborg’s case, depending on which

long houses, the outer long houses were slightly shorter than those that lay

inside the castle itself.

Regardless of the exact length measure used, the Roman foot (about 29 cm)

or the so-called Trelleborg ell (49,3 cm), all houses

in the ring fortresses have even multiples of this length in their measurements.

One has also wondered if specifically Trelleborg would have had a similar wall

from an earlier stage about contemporary with the oldest Danevirke. This is

due inter alia to some common features of these structures. The last

extension, however, is from 980/981.

We now go into features of the structure of the ringwalls themselves. They

consisted not only of sloping earthmasses as of today, but they were propped

by an inner transverse posture of fixed horizontal and vertical posts to hold

together the earthmasses, where there are also stones. The exterior of the

fortresses has been covered by a vertical oak tree palisade, connected with

the inner posture and secured with external obliquets. At the top of the wall’s

outer part was a standing nipple parapet attached to the standing postage.

Ditto a lower palissade on the inside of the castle walls. Notice that we are not

talking about stone walls. During the Viking era, no stonewalls were built in the

Nordic countries, and there was still no warfare in the surrounding area, which

demanded stone defenses.


It should be noted that ring fortresses are unique to the Danish area. There are

also Viking town walls in other parts of the Nordic region, often semi-circular

with aperture toward water and sea. These places, of course, have a different

function than the ring forts, who serve the power to hold together a kingdom,

primarily landbased.

The presence of warriors does not mean that only warriors have lived and

trained in the ring camps. There have been found traces of crafts, including

smith’s craft (not only iron for weapons but also gold and silversmithy) and in

the burial fields of ring forts there have also been found skeletons of children

and women. From Fyrkat has been found a grave with a völva or seeress with

her seeress staff. Despite the fact that ship accessibility to, for example,

Trelleborg, was limited, they have found a lot of iron nails and wood that seem

to be connected to ship building or ship maintenance.

We proceed to describe the amounts of soil filling resp. timber that was

needed to build a ring castle with all its long houses. In the case of fillings for

the actual walls, surely some will come from the excavated moat locally, but

this is not enough at all. First, the moat is not as deep as the walls are high,

and the moat usually covers only one / lesser / part of the perimeter of the

castle. A fort like Trelleborg has been estimated to have needed more than

20,000 cubic meters of earthmasses to its ringwall, which can be estimated to

a freight requirement corresponding to 1,600 lorries. As for oaktimber

requirements an estimation is about half of all oaks of the entire Zealand. If

you convert this into transport you get a stunning mileage.

Aggersborg has demanded almost four times as much of earth and gravel

masses. If we talk about timber harvesting, it should be noted that the ring

forts possibly have been planted near forests of large oak trees, which have

produced all oakwood. For a single house in Aggersborg it has been estimated

about 66 large oaks according to calculations, i.e. about 5000 large oaks for all

houses. We are probably talking about deforestation of oak forests widely

around. Why just oak wood? It is the most enduring wood, that is why.

Who were the workers?

An attempt has been made to build a single copy of the old long houses, at

Fyrkat and at Trelleborg. Each roof was covered by about 25000 roof shingles,

which ”someone” has to manufacture. From these modern attempts, it has

been concluded that it has taken about 50 full-time employed men 2.5 years to

build a single wooden house of the Trelleborg type. The raw material of oak

must be in place at the house. What was required in labour to build the fortress

itself with its various details is best left to speculation. To complete an entire

working ring castle with all its houses in just two years, including cutting and

transport of oaks, excavation and placement of moat graves, conversion of

these soil masses to the actual ring walls, retrieval of further large

earthmasses and the construction of the posts and the palisades, would have

required an extremely significant workforce. This has to be housed and fed

during the construction of the building.

So who were the workers?

Part of the work was mostly of the type of wear and tear, but other work was

rather qualified as probably the posts in the earthmasses, the palisades and

the gates. Ditto applies to the long-house buildings, these were slightly arched

as to both height and length, highest and respectively widest in the middle,

lower resp. narrower towards the house ends. Craftsmanship is required to get

this together. One could imagine thralls who did the raw job, while the qualified

work could have been carried out by a local army group, who was faithful to

Harald. One should also consider the place-name of Trelleborg, in which the

word thrall, Swe. träl (slave), is very likely a part. We should pay particular

attention to the Skåne city of Trelleborg, which is known to have at least early

medieval origins.

The skilled work could probably also have been carried out by hired farmers

from the surrounding area, but these may have been too busy with their own

livestock, their own harvest and maintenance of their own small longhouse to

be available in larger numbers. Regarding the long posts of the long houses,

they were put down in post holes in the ground, with some draining gravel bed.

The groundwater attacked these supporting posts, which had to be replaced

after about 20-25 years. Due to the better ground they were not as vulnerable

as Harald’s bridge posts across the swamps south of Jellinge. The technique

of isolating a wooden house from the soil moisture through some type of stone

foundation only arose during the Middle Ages.

We simply do not know from the sources conditions for the workers. A small

note by the 12th-century chronicle writer Svend Aggesen can support a

hypothesis about the use of warriors. Aggesen claims that the king would have

sent an army to move an extremely large stone. This could be a distorted

memory of the fact that people were commanded for a large construction work,

whose meaning they did not fully understand. Another hypothesis is that the

king would have used craftsmen from Vendland, that is to say the Slavic

populations from the southern Baltic Sea coast from somewhat east of

Denmark and eastwards.

The reason for this would be that from the eigth

century onward there are similar, more simple, wall forts of similar type to

Danish ring forts, which seem to be an advanced further development. Similar

simpler wall forts are also found in Frisland. Otherwise there are no further role

models in northwestern Europe.


Denmark has, according to various sources (including the Old Norse

literature), had connections to Vendland and archaeologically, there are some

vendic-type finds on the southernmost Danish islands, which can indicate

some Vendic / Slavic elements in the population.

Warriors of the ring fortresses and what happened

As mentioned above, the families of warriors may also have lived in the ring

camps. The warriors seem to belong to some kind of elite and seem to have

been well-fed and wealthy. One sees it from grave items in the grave fields of

the ring forts. At Trelleborg has been found a silver cup, glass beads and thin

gold plates, something archaeologically characteristic of (pre-christian) grave

goods for the higher social classes. Skeleton-wise, they were well-grown

young men whose teeth are free from tooth decay, which otherwise was

common at the time. One has investigated the presence of chemical trace

elements in teeth from these warriors. The exact levels of different trace

elements will shift between different districts and their groundwater contents

during the growth of the individual. In any case, some of these warriors seem

to have come from Vendland.

Such a descent suggests that Harald was in need of warriors who lacked

relatives in the vicinity of the ring fort (possibly all ring forts) and therefore only

had loyalty to him. One might ask how to feed wealthy and perhaps foreign

warriors in larger numbers, who did not devote themselves to the basic

farming and livestock farming industry? One possibility is that the warriors

collected debt or tax in natural products from the ring forts’ surroundings to

themselves and to Harald. The idea is perfectly reasonable from a purely

medieval point of view. But now this is still a late Iron Age culture, where there

were covenants and loyalty with their own through gifts and return gifts / return

services. Tax was something that was taken from other people, not from their

own people or their own allies.

Perhaps you can assume that Harald Blåtand and what the ring fortresses

stood for were deeply hated by the common Danes and their other chiefs?

In 983, Harald’s son, Sven Tveskägg (Svein Forkbeard), had reconquered

Hedeby and Danevirke. His star among the Danes had thus risen, to which

may have contributed that he was still ”old-fashioned” in religion, at least

according to Saxo Grammaticus i.e he still worshipped the old Norse gods. He

rebelled in the year 986 (about 5 years after the building of the ring fortresses)

against his father’s rule, probably somewhere in Jutland, and his father fled,

mortally injured, aboard a ship to Vendland, where he expected protection but

soon died. This according to a number of different sources.

Here again there is

Harald’s connection to Vendland. Harald was unable to create a geographic

realm in a later Christian or national sense, he tensioned his loyalty bonds in a

world which held them absolutely reciprocal until they burst and he himself

was killed.

The great silence

As explained above, the construction and existence of the Danish ring

fortresses must doubtless be regarded as the most remarkable feat of its time.

These forts should have given echo again and again in later chronicles.

Although politically speaking, the ring forts were abandoned within twenty

years after their building, the ruins must have been visible and startling for at

least one hundred years afterwards. Human memory and the oral tradition

should have preserved their memory into later chronicles. Though it is only

mentioned, as noted above, that Harald would have commanded his army to

move a large stone block according to Aggesen’s chronicle. Saxo Grammatic

mentions the same event. (The block would have been intended to embellish

his mother’s grave, Queen Thyra. The warriors are said to have become tired

of Harald for this trailing.) Otherwise, there is full silence about the existence of

ring forts.

The king we are talking about, who made the ring forts, is, however,

”Denmark’s first Christian king”. Normally, later Christian writers carefully

account for the actions and tales of such kings. The Norwegian Christian kings

Olaf Tryggvason and Olaf the Holy have raised a whole early medieval

literature and even ”Sweden’s first Christian King”, Olof Skötkonung, is quite

well-documented in literary sources. (Old Norse preserved text sources do not

focus on Sweden.) But not a sound about the Danish ring forts. There is a

stark contrast between the heavy presence of the ring fortresses

archeologically and their equally significant literary absence. You are tempted

to think of a conscious suppression. Now, we are talking about something that

happened at the end of the Viking era, near the verge of writing, not something

from a dim and forgotten 600’s.

Nevertheless, there is something in the Old Norse literature, which seems to

be an echo of the Danish ring forts, and it is about the jomsvikings Jómsborg.

This Jomsborg is, according to various sources, to be located to (noticeably!)

Vendic area, to Wollin at the Oder estuary. We are not talking of ring forts or

fortifications in Denmark itself. There is a proven archeological Scandinavian

presence during this time at Wollin, though not of a kind that implies

dominance. Jómsvíkinga saga (written in the late 1100s, that is, when the

crusades to the ”holy land” had begun) depicts that the Jomsvikings in their

Jomsborg would have been their own political unity at the level of Danish or

Slavic kings, just as the crusaders were in their time.

Jomsborg is portrayed as a stone castle on the seashore with a gate that could

be opened and let in guest ships inside the castle; after negotiation with the


Jomsvikings and their leaders

The jomsviking chieftain is designed as an absolute ruler through the strict

warrior laws that prevailed there. These are reminiscent of the rules

of order in a crusader order. What has just been said

about Jomsborg is, of course, an anachronistic fantasy story and no such

archaeological Jomsborg is existant. Nevertheless, the wounded Harald fled to

Vendland. Adam of Bremen says that Harald fled on a ship to the Slavic city,

called Jumne and died there from his wounds. Saxo says that Harald fled to

”Jullin”. According to the Ágrip af sögu Danakonunga (story of kings of the

Danes), Harald was wounded by his own son and ”flýði til Jómsborgar í

Vindland”, escaped to Jomsborg in Vendland. And an independent source,

Encomium Emmae Reginae, says that Harald was defeated by his son and

had to flee wounded to the Slavs.

It is clear that Harald had friendly relations with Jumne / Wollin, but also that

there were Vends / Slavics who ruled here, not any specific Jomsvikings or

even Danes. A deviant voice about Wollin is, however, Svend Aggesen’s

chronicle, which seems to indicate that it would be Harald Blåtand, who would

have founded Wollin.

The Jomsvik saga also tells us that Danes and Jomsvikings together would

have drawn to Norway and lost the battle 986 at Hjörungavåg, which is

considered a historic battle. Two named Jomsborg leaders, Sigvaldi and Búi,

are also mentioned in other Norwegian descriptions of this kind, but not as

Jomsvikings but as Danes. In addition, there are scattered tales in the sagas

of the Norse kings that single Vends would have been in Norway. Vends and

Norse people from Norway are not near neighbours, nor do they speak the

same language.

There are few other possibilities than that the Jomsvikinga saga’s (and some

other Norse texts’) Jomsborg has a distorted reality background and as model

has the Danish Ring Forts. The actual reality of the ”jomsvikings” were then

the Vendian warriors who, according to dental analysis, at least partially

populated these Danish ring camps. They can certainly have had special rules

of camps and Saxo Grammaticus discusses generally special army regulations

in Denmark. However, hardly any ”rules of Order”, as stated by the jomsviking


If we are to designate a single ring castle as a model for the Jomsborg fairy

tale, then we need to highlight Aggersborg. This is the northernmost ring castle

in view of the ”jomsviking” participation in the battle of the Norwegian

Hjörungavåg. Aggersborg could have served Danish interests in southern

Norway. Aggersborg differs from other ring fortresses. It is many times larger

than the rest, it is the northernmost and it seems to have had full access to

larger ships through its location. It would therefore be more maritimely

offensive than other ring forts. These appear defensively oriented and concern

rule over land.


We could easily have developed many more themes. That the Danish Harald

Blåtand (Blue Tooth) really had land interest in southern Norway at this time, is

apparent from his rune stone: ”The Harald who won all Denmark and Norway

and made the Danes Christian”. The same is true of Snorri Sturluson’s

Heimkringla or the Sagas of the Norse kings. The purpose of this paper is,

however, merely to illustrate what the Danish ring fortresses involved locally.

We simply stop with this finding and leave the major politics of that time, along

with its later historical (Old Norse) interpretations.




Facts and Fancy in Jómsvík saga, by Leszek P. Słupecki (Polish Academy of

Sciences, Institute of Archeology and Ethnology), Saga Conference 2006

Illustrerad vetenskap och historia, a special edition: Yearbook 2014 from

January 2015

Jómsvíkinga saga in suitable translation, online, try www.heimskringla.no

Saxo Grammaticus Danmark’s Krønike – Saxonis Grammatici Historia Danica

was printed in Paris 1514 by Kristjern Pedersen. Translated by Fr. Winkel

Horn to modern Danish from Latin. Available to search online.

Vikinger i krig (Vikings in War), by Kim Hjardar and Vegard Vike, Spartacus

Publisher, 2011. ISBN 978-82-430-0475-7