Trelleborg is believed to have been ordered by King Harald Bluetooth in the year 980 AD and it might have commanded the Great Belt and its sea traffic, between the islands of Zealand and Funen.

Along with four other Viking ring fortresses in denmark, Trelleborg was inscribed on the UNESCO World Heritage List in 2023 because of its unique architecture and testimony to the strategic and military power of the House of Knýtlinga (Jelling Dynasty).

[citation needed] The circular main castle was surrounded by a 5 m high rampart, 17.5 m wide at the base and with a diameter of 137 m. The outer walling was made of oak.

The 14 longhouses of the bailey, each 26.33 meters long, were placed with their axis through the length of the buildings pointing to the center of the main castle.

Most of the finds at Trelleborg, reflects a relatively peaceful daily life here and includes every day utensils such as pottery, locks, keys, fittings, knives, whetstones, combs, weaving weights, scissors and needles.

The slain soldiers were quickly buried in the massgraves, many of them showing deep cuts and lethal wounds from close combat weapons.

Strontium analysis of the skeletal remains has revealed that a larger part of the dead, originated from what we now know as Norway and Poland and are therefore thought to have been foreign mercenaries, stationed here as castle guards.

This discovery falls in line with Harald Bluetooth's strong alliance with the Obotrite Slavs through his marriage with Tove, daughter of prince Mstivoj.

The landscape surrounding the Trelleborg has changed considerably since the Viking Age, due to a land rise known as post-glacial rebound, affecting this part of the world, and the previous swamp has shrunk to a small bog, while the two rivers of Vårby Å and Tude Å, has reduced to narrow streams.

The municipality has also initiated a large nature restoration project of the entire river valley of Tude Å, including the trelleborg.

[2][6] In the book Eaters of the Dead by Michael Crichton, Trelleborg (mentioned as Trelburg) was a stop on Ibn Fadlan's journey.

In the book “Jomsviking” by Norwegian author Bjørn Andreas Bull-Hansen, Trelleborg figures as the base of Danish King Svein Forkbeard at the time of the battle of Svolder.