There are a total of five confirmed Viking ring fortresses at present, located in Denmark (although sites in Sweden and across Northern Europe have similar construction).

In 2023, the five Danish forts were inscribed on the UNESCO World Heritage List because of their unique architecture and testimony to the military power of the Jelling Dynasty.

[5] At the Firth of Schlei lay Hedeby, known in the contemporary literary sources as Schleswig, where the Danevirke complex of fortifications stretched across the foot of the peninsula, holding back the hostile hosts from entering the territory, as well as providing a safe trade route via Ejderen from the North Sea coastline into Hedeby and the Baltic Sea.

The fortresses establish a string of strategic points stretching from Aggersborg at the north of Jutland southward across Funen to end in Borgring at the east coast of Zealand.

[6] They have been dated to the reign of Harold Bluetooth, who held sway until c. 985, where he was ousted by his son Sweyn Forkbeard, who eventually conquered the Anglo-Saxon kingdom of Æthelred the Unready some years later.

[2] The trelleborgs had similar design,[17] "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.

[citation needed] 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.

[23] At the turn of the century much debate had surrounded the ring fortresses,[24] particularly with emphasis on whether or not to add Nonnebakken to the list of recognized trelleborgs.

Nonnebakken was accepted as late as 2017,[25] though at that time the archaeologist Mads Runge concluded there were no signs of buildings inside the rampart.

[26] When geo-radar technology was applied the following year, evidence of structures inside the fort was found, affirming the site's classification as a Viking ring fortress.