There are five main and well-known [[Viking ring fortress|ring fortresses]] in SwedenDenmark; they have been dated to the reign of Harald "Bluetooth" of Denmark (died 986 CE). The fortresses are Trelleborg, Aggersborg, Fyrkat, Nonnebakken,and Borgring. Of these five, Trelleborg has been the best preserved and is unique as it contains a fortified ward. <ref>{{Cite web |title=UNESCO |url=https://en.natmus.dk/museums-and-palaces/trelleborg/unesco/ |access-date=2023-12-01 |website=National Museum of Denmark |language=en}}</ref> The fortresses made use of their surrounding geography to enhance defensive strength. Curiously, the true nature and purpose of these buildings are not well understood as there was a lack of documentation surrounding the buildings. The most commonly accepted theory is that these fortresses were built by Bluetooth to not only show his strength and sovereign power but as an attempt to combine and establish a proper and unified kingdom under his rule. The complexity and size of the structures show that there had to be an extreme centralized power to establish them and keep them functioning. However, due to the turmoil of the time period with various power struggles, these structures were most likely very difficult to maintain, leading to them falling into disuse within a mere decade. <ref>{{Cite web |last=Centre |first=UNESCO World Heritage |title=Viking-Age Ring Fortresses |url=https://whc.unesco.org/en/list/1660/ |access-date=2023-12-01 |website=UNESCO World Heritage Centre |language=en}}</ref>
The structures themselves are built in a specific geometric way to ensure defensive power and strength. Each fortress was built in a precise circular shape hence "ring fortress". There were four main gates located equidistant from each other with ramps located with each gate connected by roads. There is a main circle road that connects with these axial roads. Within the fortress is a number of [[longhouse]]s that are arranged to compliment the geometric structure of the ring fortress. Outer ramparts were created from wood and dirt and The largest of the fortresses was Aggersborg. <ref>{{Cite web |last=Price |first=Michael |date=August 16, 2017 |title=Thousand-year-old Viking Fortress Reveals Technologically Advanced Society |url=https://www.science.org/content/article/thousand-year-old-viking-fortress-reveals-technologically-advanced-society}}</ref>
