Viking ring fortress

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, as 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]

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] Søren 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 Swedish 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 in Denmark. 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 at the time was obscured by farmland (it has subsequently been partially re-established)

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.

Denmark and Sweden are currently applying for admission of the Viking ring fortresses as UNESCO World Heritage sites.[10] Five of the Danish ring fortresses were listed in the Viking-Age Ring Fortresses World Heritage Site in 2023.[11]

Map of Viking ring fortresses

Confirmed

• Aggersborg near Aggersund, Denmark.
• Borgeby north of Lund in Scania, Sweden.[12]
• Borrering near Køge, Denmark.[13][14][15]
• Fyrkat near Hobro, Denmark.
• Nonnebakken in Odense, Denmark.[16][17]
• Trelleborg near Slagelse, Denmark.
• Trelleborgen in Trelleborg, Scania, Sweden.[18][19]

Suspected

• Helsingborg in Scania, Sweden.[1]
• Rygge (59.38712°N 10.7062°E / 59.38712; 10.7062 (Rygge)) in Østfold, Norway.[9]

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 other 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. All 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."[20]

Dating by dendrochronology has found the wood used for the construction of Trelleborg to have been felled in the autumn of 980 and thus being used for building presumably in the spring of 981. The rather short construction time and the complete lack of any signs of maintenance indicate only a 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 are so similar that it is probable that they were built to a single plan.

• 
  Aerial image of Fyrkat.
• 
  Recreation of a typical building at Fyrkat.
• 
  Recreation of the fortress walls at Trelleborg.

• Circular rampart
• Ringfort
• Ravning Bridge, a contemporary large-scale structure in Denmark.