Viking ring fortress
A Viking ring fortress, or Trelleborg-type fortress, is a type of circular fort of a special design, built in Scandinavia in the Viking Age. They are also known simply 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 them have been dated to the reign of Harold Bluetooth of  Denmark (died 986). The fort in Borgeby 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. Etymology. 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 "" 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. List of "trelleborgs". 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. Also in Lund, Scania, the street topography indicates a Trelleborg. Comparison of the seven fortifications. The ring castles and the contemporary Ravning Bridge over Vejle Rivertogether 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. 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." 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 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, 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. 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 "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. 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. 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. In Scandinavia, fortifications of a similar design and date have been found around some old towns, like in Aarhus for example, but they lack the perfect circular geometry. Scholars have offered various hypotheses on the functions served by the fortresses. 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. Andres Dobat has argued that the fortresses were means of control and administration by Harald Bluetooth. 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. Speculation. In 1990, Danish hobby pilot Preben Hansson observed that the Trelleborgs at Aggersborg, Fyrkat, and Slagelse , and a ringwall at Eskeholm (Samsø, ), appear to be aligned, a kind of Ley line. The theory that the fortresses were planned by prehistoric aviators has been popularized by Erich von Däniken.