Nonnebakken belongs to a set. Around 980 the Danish crown built a series of near-identical circular fortresses across the kingdom, geometrically precise, and the archaeology puts Nonnebakken's construction between 980 and 1000. It went up during the reign of Sweyn Forkbeard, who drove his father Harald Bluetooth from the country in 986 or 987 — the old king took refuge with the Jomsvikings at Wollin, in what is now Poland, and died soon after of wounds from the fighting. The site was chosen for control rather than comfort. From the hill, whoever held the fortress held the passage of the Odense Å running past its foot, and with it the approach to the settlement on the opposite bank.

The Viking name for the place is lost. The one it carries comes from the Benedictine nuns who occupied the hill in the twelfth and thirteenth centuries, long after the ramparts had stopped meaning anything military. At the end of the twelfth century the community moved out to build a new church at Dalum, to the south-east, now a suburb of Odense. Their old buildings sank into the ground and stayed there; excavation in 2002 turned up parts of the nunnery. Some of the ditches that ringed the fortress had, by then, been deliberately filled — and the latest filling dates to the nunnery's time, meaning the nuns were tidying away a fortification already two centuries old.

The single best picture of Nonnebakken predates archaeology by three and a half centuries. The earthworks are visible in the panorama of Odense engraved in 1593 and published as plate 30 of volume five of Georg Braun and Frans Hogenberg's Civitates Orbis Terrarum, completed in 1598 — the engraver worked from information supplied by the Holstein nobleman Heinrich Rantzau. The image shows the ring south of the river, opposite the medieval city, still legible as a shape in the landscape. Then came the nineteenth century. A building for the local Odd Fellow lodge went up on the site in the late 1800s and did heavy damage to what lay beneath, and by the early 1900s the last of the ramparts had been levelled.

Finds were reported from the hill as early as 1775, when construction work turned up an arm-ring later known as Odin's ring, and again in 1889. But the extent of the whole structure was not established until 1953. Every advance since has come from someone digging for an unrelated reason. Cable-laying in 1988 exposed a filled ditch with a pointed profile, at least four metres wide and two deep, with a berm of ten to twelve metres; a second pointed ditch, eight metres wide and about four deep, was traced to the north-east and north-west. Heating-pipe works in 1995, 1997 and 1998 cut through a ditch eleven metres wide and three metres deep — the trench that gave up the spade. Every fragment of the plan now known has been read off the walls of somebody else's excavation.

In 2023 Nonnebakken was inscribed on the UNESCO World Heritage List together with four other Danish ring fortresses — Aggersborg, Fyrkat, Trelleborg and Borgring — as evidence of the strategic and military reach of the House of Knýtlinga, the Jelling dynasty. It is an odd sort of monument to visit. There is no rampart to climb and no gate to walk through, only an address in a modern city and the knowledge that a precisely engineered circle of earth once stood exactly here. Odense built over it before anyone understood what it was, which makes Nonnebakken a useful reminder: the most consequential things underneath a city are usually found by accident, in a trench dug for something else.