Viking ring fortress

For other uses, see Trelleborg (disambiguation).
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 is a type of circular fort of a special design, built by the Vikings in the Viking Age. They are also known as Trelleborgs. All Trelleborgs have a strictly circular shape, with roads and gates pointing in the four cardinal directions. These common structures are sometimes partially encircled by advanced ramparts, but these additions are not always circular.

There are a total of seven known Viking ring fortresses at present, located in Denmark and Scania, Sweden. Most of the seven Trelleborgs have been dated to the reign of the Harold Bluetooth of Denmark (died 986). The fort in Borgeby[1] has been dated to around 1000 AD, so it is possible that it too, was built by the same king.

Denmark and Sweden are currently applying for admission of the Viking ring fortresses as UNESCO World Heritage sites.[2]


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 Danish word is træl) and borg means fortress or city. But the word trel (pl. trelle) is also a plausible explanation and relates to the wooden staves, covering both sides of the protective circular walls.[3]

List of trelleborgs

Sites of six of the Viking ringcastles.

The existence of a Viking ring fortress at Helsingborg in modern Sweden, 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.[9]

Comparison of the seven fortifications

Name Inner
Number of
Length of
Position Year of discovery Year of construction
Aggersborg 240 m 11 m 48 32.0 m 56°59′43.6″N 9°15′17.8″E / 56.995444°N 9.254944°E / 56.995444; 9.254944 (Aggersborg) 980
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′10.97″N 12°7′19.01″E / 55.4697139°N 12.1219472°E / 55.4697139; 12.1219472 (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′32.10″N 10°23′17.35″E / 55.3922500°N 10.3881528°E / 55.3922500; 10.3881528 (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 / 55.37624°N 13.14756°E / 55.37624; 13.14756 (Trelleborg, Sweden) 1988 c. 800

The ring castles and the contemporary Ravning Bridge over Vejle River  together with minor bridges erected on Zealand (Bakkendrop bridge between Gørlev Tissø and Risby bridge by Præstø) and Lolland (over Flintinge river)  differ clearly from others from the Viking Age. Unlike other ring castles from the period the ring castles 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.

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

All five 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."[10]

In spite of searches no real parallels have been found in the rest of Europe. On the coasts of the Netherlands and Belgium there are ring castles with certain points of resemblance and on the island Walcheren 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.[11] 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.

Datings by dendrochronology have found the wood used for the construction of Trelleborg (near Slagelse) 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 Trelleborg 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.

Around 974 the Danish Viking king Harald Bluetooth lost control of the Danevirke and parts of Southern Jutland to the Germans. 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.

Another theory is that the ring castles were boot camps for the troops used by Sweyn Forkbeard in his attack on England. Sweyn and his men sacked London in 1013.

Fortifications of a similar design and date, has been found around some old towns, like in Aarhus for example, but without the perfect circular geometry.[12]


In 1990, Danish hobby pilot Preben Hansson observed that the Trelleborgs at Aggersborg, Fyrkat, and Slagelse, and a ringwall at Eskeholm (Samsø, 55°53′08″N 10°39′08″E / 55.88543°N 10.6522°E / 55.88543; 10.6522 (Eskholm, Samsø (not a trelleborg))), appear to be aligned, a kind of Ley line.[13] The theory that the fortresses were planned by prehistoric aviators has been popularized by Erich von Däniken.[14][15]

See also


  1. The second circular fort "Trelleborg" found in Sweden Archived June 16, 2004, at the Wayback Machine.
  2. "The Viking Era's Trelleborg fortresses". Danish Agency for Culture. 20 May 2010. Retrieved 29 May 2015.
  3. Janne Bøje Andersen (2010). "Veje ind i arkæologien" (PDF) (in Danish). p. 66. ISBN 978-87-993972-0-4. Retrieved 7 September 2014.
  4. "Two Scanian ring forts from the Viking Age" (in Danish). Poul Erik Lindelof. Retrieved 6 September 2014. A private homepage. Sourced.
  5. "Enigmatic Viking Fortress discovered in Denmark". Danish Castle Center. Retrieved 6 September 2014.
  6. "Borgerring". Fund og Fortidsminder (in Danish). Danish Agency for Culture. Retrieved 6 September 2014.
  7. "Trelleborgen". Trelleborg Municipality. Retrieved 16 September 2014. Official Homepage.
  8. "The Trelleborg" (in Swedish). Trelleborg Municipality. 2012. Retrieved 16 September 2014.
  9. Margareta Weidhagen-Hallerdt (2009). "A possible ring fort from the late Viking period in Helsingborg" (PDF). Current Swedish Archaeology. Retrieved 6 September 2014. line feed character in |author= at position 10 (help)
  10. A. Forte, R. Oram, and F. Pederson. Viking Empires. 1st. ed. Cambridge: Cambridge University Press, 2005 ISBN 0-521-82992-5 p. 180.
  11. For example, Warham Camp.
  12. "The Vikings' Aros - The Ramparts". The Viking Museum (in Danish). Moesgård Museum. Retrieved 6 September 2014.
  13. Hansson, Preben: Und sie waren doch da (1990) Bayreuth, ISBN 3-7770-0419-7
  14. Däniken, Erich von: Die Steinzeit war ganz anders (1993) Munich, ISBN 3-442-12438-7
  15. Däniken, Erich von: Auf den Spuren der Allmächtigen (1993) Munich, ISBN 3-570-01726-5
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