It is the site of one of Denmark's six former Viking ring castles,[1][2] built during the reign of Sweyn Forkbeard, who had forced his father Harold Bluetooth to leave the country and seek refuge with the Jomsvikings on Wollin (modern Poland) around 975.

At the end of the 12th century, the nuns left the site to build a new church in Dalum to the southeast, now a suburb of Odense.

The earthwork ramparts can be recognized in the "panorama of Odense" in the medieval tome Civitates Orbis Terrarum from Georg Braun and Frans Hogenberg.

The archaeological remains of Nonnebakken took heavy damage during the late 19th century, when a building for the Odd Fellow lodge was constructed on the site.

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