Viking ring castles
A Viking ring fortress, ring fort or Trelleborg-type fortress is a type of circular fort of a special design, built by the Vikings 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 the seven trelleborgs have been dated to the reign of the 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.
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 ) 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.
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.
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.
...
Wikipedia