Viking ring fortress

Trelleborg is a collective name for six Viking Age circular forts, located in Denmark and the southern part of modern Sweden. Five of them have been dated to the reign of the Harold Bluetooth of Denmark (d. 986). The fort in Borgeby [ [http://viking.hgo.se/Files/Sweden/Skane/Borgby.html The second circular fort "Trelleborg" found in Sweden] ] has been dated to the vicinity of 1000 AD, so it is possible that it too, was built by the same king.

This type of fortification is also named after the first discovered example; Trelleborg near Slagelse, excavated 1936-1941). All "trelleborgs" have a strictly circular shape. This structure may also be (partially) encircled by an advanced rampart but this part of the structure is not necessarily circular.

List of "trelleborgs"

*Aggersborg near Limfjorden, Denmark.
*Borgeby north of Lund at Lödde Å in Skåne, modern Sweden.
*Fyrkat near Hobro, Denmark.
*Nonnebakken in Odense, Denmark.
*Trelleborg near Slagelse, Denmark.
*Trelleborg in Trelleborg, Skåne, modern Sweden.
* [http://arkeologi.blogspot.com/2005/03/en-trelleborg-i-rygge.html Rygge] , Østfold, Norway?

Traditionally, the name "Trelleborg" has been explained as "a fort built by slaves" (the Danish word for "slave" being "træl"), but the word trel (pl trelle) in a more plausible explanation. This relates to the wooden staves covering both sides of the protective circular walls.

=Comparison of the six fortifications=

Name	Inner diameter	Rampart width	Number of houses	Length of houses
Aggersborg	240 m	11 m	48	32.0 m
Borgeby	150 m	15 m
Fyrkat	120 m	13 m	16	28.5 m
Nonnebakken in Odense	120 m
Trelleborg near Slagelse	136 m	19 m	16	29.4 m
Trelleborg in Trelleborg	125 m

The ring castles and the contemporary bridge over Ravning Enge/Vejle Å (Vejle river)ndash together with minor bridges erected on Zealand (Bakkendrop bridge between Gørlev Tissø and Risby bridge by Præstø) and Lolland (over Flintinge river)ndash 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.

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" [Fortehad, Oram and Pedersen, "Viking Empires", Cambridge Uni. Press (2005), pg. 180]

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. [ [http://new.edp24.co.uk/content/Hidden/story.aspx?brand=EDPOnline&category=Hidden&tBrand=EDPOnline&tCategory=Hidden&itemid=NOED19%20Aug%202006%2015%3A16%3A36%3A180 for example Warham Camp] ] These generally date though from around the time of the roman conquest of celtic Britain and had been laying 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 fall 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.

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