Nonnebakken (literally, "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 the Benedictine Nunnery located on the site at earlier times. To 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 earthworks can still be recognized in the panorama of Odense in the " Civitates Orbis Terrarum" from Georg Braun and Frans Hogenberg. The engraving is in volume 5, that was finished in 1598 (although first published in 1612-18 in Cologne?), on plate 30, that was made in 1593 from information contributed by 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 the fort took heavy damage when a building for the Odd Fellow lodge was constructed on the site during the late nineteenth century. The site was excavated by the Fyns Stiftsmuseum. The castle had a diameter of 120 m and dates to 980-1000, similar to the other Viking ring castles.

Excavation and finds
Although finds have been reported from 1775 and 1889 the extent of the whole structure was first determined in 1953. The next major gain in Information was when in 1988 trenches where dug for laying cables. A ditch with pointed bottom was observed during the excavations. It was at least four meters wide and two meters deep with a berm of maximal 10 to 12 meters. The later filling could be dated to the time of the abbey. The profil of a ditch with pointed bottom eight meters wide and some four meters deep was excavated in the northeast and the northwest. During excavations prior to larger digings for heating pipes in 1995/97/98 a ditch eleven meters wide and three meters deep was observed. A spade made of oak was found that was dated with dendrochronology to the "functioning" time of the structure. Excavations in 2002 revealed parts of the abbey.

(Danish) Kronborg Christensen, John (1973). Vikingetidens langhuse på Trelleborg, Aggersborg, Fyrkat og Nonnebakken (83 pages). Copenhagen: Kunstakademiets Arkitektskole. (Danish) Thrane, Henrik (1987). Nonnebakken. Odenses forsvundne vikingeborg, 3rd revised ed. (28 pages). Odense: Fyens Stiftsmuseum.