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 ( died 986 ) . The fort in Borgeby 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 " . 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 ) is a more plausible explanation . This relates to the wooden staves covering both sides of the protective circular walls . Comparison of the six fortifications . The ring castles and the contemporary bridge over Ravning Enge / Vejle Å ( 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 moat s 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 . " 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 . 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 . 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 .