Wikipedia
Nonnebakken (literally, "The Nun Hill") is a hill in Odense, Denmark. It is the site of one of Denmark's six former Viking ring castles, built during the reign of Sweyn Forkbeard, who had forced his father Harold Bluetooth to leave the country and seek refuge by the Jomsvikings on Wollin (modern Poland) around 975. The fort enabled its occupier command of the Odense River passing next to the hill.
The name refers to a benedictine nunnery located here in earlier times. 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 panorama appears as plate 30 in Volume 5, finished in 1598. Plate 30 was engraved in 1593, based on information from Heinrich Rantzau or from sketches he gave in order. It also appears on the panorama of Odense published by Braunius in 1593.
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. In the 20th century, scientific excavations led by Fyns Stiftsmuseum revealed that the circular castle had an inner diameter of 120 m and was built in the years 980-1000, similar to the other Viking ring castles.