Aerial view of the Viking ring fortress of Trelleborg, near Slagelse in Denmark. This was the first rediscovered Viking ring fortress, and the geometry is clearly visible.

A Viking ring fortress, or Trelleborg-type fortress,[1] is a type of circular fort of a special design, built in Scandinavia during the Viking Age. Collectively, they may also be known as trelleborgs. These fortresses have a strictly circular shape, with roads and gates pointing in the four cardinal directions. They are sometimes partially encircled by advanced ramparts, though not always circular.

There are a total of seven known Viking ring fortresses at present, located in Denmark and Scania, Sweden. Many of them have been dated to the reign of Harold Bluetooth of Denmark, c. 980. Their exact purpose is subject to debate.


This specific type of fortification was named after the first discovered example: Trelleborg near Slagelse, excavated in the years 1936–1941. Traditionally, the name trelleborg has been translated and explained as ″a fortress built by slaves″, since the Old Norse word for slave was thrall (The modern word is træl in Danish and träl in Swedish) and borg means fortress or city. The word trel (pl. trelle) is also a plausible explanation and relates to the wooden staves, covering both sides of the protective circular walls.[2]


Around 974 the Danish Viking king Harald Bluetooth lost control of the Danevirke and parts of Southern Jutland to the Saxons. The entire complex of fortifications, bridges and roads which were built around 980 are presumed by some to be Harald's work, and part of a larger defensive system. Fortifications of a similar design and date have been found around other old towns in Scandinavia, like in Aarhus for example, but they lack the perfect circular geometry of the distinct ring fortresses.[3] Most of the fortresses have been dated to the reign of Harald Bluetooth, which lasted from c. 958 until 986. The fort in Borgeby, however, has been dated to c. 1000, though as the precise date is undetermined, it is possible that it was also constructed as part of his fortification push.[4][citation needed]

The precise purpose of the fortresses, however, is unknown. Some historians argue that they functioned as military barracks or training grounds by Sweyn Forkbeard. However, it is more likely that they were intended as defensive strongholds along strategic trade points and/or administrative outposts of the budding state.[5][6] Soren Sindbæk has offered the hypothesis that the fortresses allowed local populations to seek shelter within the fortress walls against an enemy while waiting for assistance from friendly forces from afar; this means that the fortresses helped Harald Bluetooth to control vast territory and send his army to a particular part of his territory without worrying that the undefended parts would be conquered or plundered.[7] Others have debated whether the fortresses were defensive structures, military strongholds, or primarily served as barracks, as well as the economic, religious, and symbolic significance of the fortresses.[8]

Many of the fortresses were abandoned by the end of the Viking Era. Several were enveloped by other settlements, such as Nonnebakken which now lies underneath Odense, and Trelleborgen under the city of Trelleborg. Others, were forgotten and receded into the landscape. The modern rediscovery of these sites began in the 1930s, with the excavation of Trelleborg. Since then, a total of seven sites have been officially recognised at Viking ring fortresses. A possible eighth site in Helsingborg was suggested in 2009 after archaeological excavations since 1987. The Helsingborg ring fort might have been the largest of them all, at a diameter of 270 m.[1] Another site in Rygge has also been proposed, but not confirmed.[9]

Satellite image showing the ring outline of Borgring on the landscape, which has been obscured by farmland.

Similar structures have been found throughout Northern Europe, particularly in Ireland, but none of them have the same strict and precise geometrical design of the Scandinavian ring fortresses. On the coasts of the Netherlands and Belgium there are ring castles with certain points of resemblance. On the island Walcheren, for example, there are the remnants of a castle with gateways in the four points of the compass, combined with streets. Similar forts can be found in England, such as Warham Camp. These generally date though from around the time of the Roman conquest of Celtic Britain and had been lying in ruins for hundreds of years prior to the building of the Viking ring forts.[citation needed]

Denmark and Sweden are currently applying for admission of the Viking ring fortresses as UNESCO World Heritage sites.[10]

List of viking ring fortresses

Map of Viking ring fortresses



Comparison of the seven fortifications

Aerial view of the Viking ring fortress of Aggersborg. The similarity in design with Trelleborg near Slagelse, is clearly evident.

The viking ring fortresses differ clearly from others types of fortifications from the Viking Age. Unlike other circular forts from the period, the ring fortresses which follow the Trelleborg model are constructed after a strictly geometrical plan and measured with the Roman foot. The pointed bottoms of the moats is another element borrowed from the Ancient Romans.[citation needed] All of such fortresses had similar designs, "perfectly circular with gates opening to the four corners of the earth, and a courtyard divided into four areas which held large houses set in a square pattern."[19]

Datings by dendrochronology have found the wood used for the construction of Trelleborg to have been felled in the autumn of 980 and thus being used for building presumedly in the spring of 981. The rather short construction time and the complete lack of any signs of maintenance indicate an only short use of the buildings, maybe five years but hardly more than twenty. The others have been dated to roughly the same time. Fyrkat may be a little older, Aggersborg somewhat younger. Not enough has been found at the other sites for a precise dating but the construction and layout of the trelleborgs at Slagelse, Fyrkat, Aggersborg, Nonnebakken under Odense and the fort under modern Trelleborg in Sweden is so similar that it is believed most probable that they were conceived by a single mind.[citation needed]

Name Inner
Number of
Length of
Position Year of discovery Year of construction
Aggersborg 240 m 11 m 48 32.0 m 56°59′44″N 9°15′18″E / 56.99556°N 9.25500°E / 56.99556; 9.25500 (Aggersborg) 975–980[20]
Borgeby 150 m 15 m 55°45′05″N 13°02′12″E / 55.75139°N 13.03667°E / 55.75139; 13.03667 (Borgeby) 1997
Borrering 122 m 10–11 m 55°28′11″N 12°7′19″E / 55.46972°N 12.12194°E / 55.46972; 12.12194 (Vallø Borgring) 2014 (1875)
Fyrkat 120 m 13 m 16 28.5 m 56°37′24″N 9°46′14″E / 56.62333°N 9.77056°E / 56.62333; 9.77056 (Fyrkat) 1950 980
Nonnebakken 120 m 55°23′32″N 10°23′17″E / 55.39222°N 10.38806°E / 55.39222; 10.38806 (Nonnebakken) 1953 980–1000
Trelleborg 136 m 19 m 16 (30) 29.4 m 55°23′39″N 11°15′55″E / 55.39417°N 11.26528°E / 55.39417; 11.26528 (Slagelse) 1936 981
Trelleborgen 112 m 55°22′34″N 13°08′51″E / 55.3762°N 13.1476°E / 55.3762; 13.1476 (Trelleborg, Sweden) 1988 c. 800


See also


  1. ^ a b c Margareta Weidhagen-Hallerdt (2009). "A possible ring fort from the late Viking period in Helsingborg" (PDF). Current Swedish Archaeology. 17. Retrieved 6 September 2014.
  2. ^ Janne Bøje Andersen (2010). Veje ind i arkæologien (PDF) (in Danish). p. 66. ISBN 978-87-993972-0-4. Retrieved 7 September 2014.
  3. ^ "The Vikings' Aros – The Ramparts". The Viking Museum (in Danish). Moesgård Museum. Retrieved 6 September 2014.
  4. ^ The second circular fort "Trelleborg" found in Sweden Archived June 16, 2004, at the Wayback Machine
  5. ^ Brown, Hannah; Goodchild, Helen; Sindbæk, Søren M. (2014). "Making Place for a Viking Fortress. An archaeological and geophysical reassessment of Aggersborg, Denmark". Internet Archaeology. 36 (36). doi:10.11141/ia.36.2.
  6. ^ Dobat, Andres Siegfried (2009). "The State and the Strangers: The Role of External Forces in a Process of State Formation in Viking-Age South Scandinavia (c. ad 900-1050)". Viking and Medieval Scandinavia. 5: 65–104. doi:10.1484/J.VMS.1.100674. ISSN 1782-7183. JSTOR 45019120.
  7. ^ Sindbæk, Søren (January 2020). "Borgring and Harald Bluetooth's Burgenpolitik". Viking Encounters. Proceedings of the Eighteenth Viking Congress, Denmark, August 6-12, 2017.
  8. ^ Price, T. Douglas (2015). Ancient Scandinavia: An Archaeological History from the First Humans to the Vikings. Oxford University Press. p. 350. ISBN 978-0-19-023199-6.
  9. ^ a b Stylegar, Frans-Arne H. (29 March 2005). "En trelleborg i Rygge?". Arkeologi i nord (in Danish). Retrieved 29 July 2022.((cite web)): CS1 maint: url-status (link)
  10. ^ "The Viking Era's Trelleborg fortresses". Danish Agency for Culture. 20 May 2010. Retrieved 29 May 2015.
  11. ^ "Two Scanian ring forts from the Viking Age" (in Danish). Poul Erik Lindelof. Retrieved 6 September 2014. A private homepage. Sourced.
  12. ^ "Enigmatic Viking Fortress discovered in Denmark". Danish Castle Center. Retrieved 6 September 2014.
  13. ^ "Borgerring". Fund og Fortidsminder (in Danish). Danish Agency for Culture. Retrieved 6 September 2014.
  14. ^ Lund, Julie; Sindbæk, Søren M. (2021-05-15). "Crossing the Maelstrom: New Departures in Viking Archaeology". Journal of Archaeological Research. 30 (2): 169–229. doi:10.1007/s10814-021-09163-3. ISSN 1573-7756.
  15. ^ Runge, Mads (2017). "New archaeological investigations at Nonnebakken, a Viking Age fortress in Odense". Hansen, J. & M. Bruus: The Fortified Viking Age. 36th Interdisciplinary Viking Symposium, S. 44-59.
  16. ^ Runge, Mads (2019-07-03). "Revitalising the Danish Viking Age Ring Fortress Nonnebakken, Odense, Denmark". Landscapes. 20 (2): 98–119. doi:10.1080/14662035.2020.1861726. ISSN 1466-2035. S2CID 232080910.
  17. ^ "Trelleborgen". Trelleborg Municipality. Retrieved 16 September 2014. Official Homepage.
  18. ^ "The Trelleborg" (in Swedish). Trelleborg Municipality. 2012. Retrieved 16 September 2014.
  19. ^ A. Forte, R. Oram, and F. Pederson. Viking Empires. 1st. ed. Cambridge: Cambridge University Press, 2005 ISBN 0-521-82992-5 p. 180.
  20. ^ Ten Harkel, Letty (June 2015). "Aggersborg. The Viking-age settlement and fortress". Antiquity. 89 (345): 759–760. ProQuest 1761147618.