The other trelleborgs are compact. Trelleborg itself measures 136 metres inside the rampart, Fyrkat 120, Nonnebakken 120, Borgring 122. Aggersborg is 240 — 288 metres to the outer edge of the earthwork — and the difference is not incremental but categorical. Four roads met at the centre in a cross and ran out to the cardinal points, tunnelled beneath the rampart so the circle was never broken. The wall itself stood about four metres high, built of soil and turf, stiffened and faced with oak. Eight metres beyond it ran a ditch roughly 1.3 metres deep. Inside, forty-eight longhouses stood in four quadrants, twelve to each, every one aligned north–south or east–west. No two were placed casually. The geometry is the point: this is a building laid out by people who could survey a landscape and hold a line.

None of the houses survive above ground, but archaeologists found where their walls stood, and the outlines are unmistakable — 32.5 metres long, 8.5 across, with curved walls and a curved roof ridge, the shape of an overturned ship. The same silhouette shows up on the Cammin casket, a house-shaped reliquary, and on house-shaped tombstones in England. Inside each was a long hall of around nineteen metres with smaller rooms at either end. A single one of these houses took an estimated sixty-six mature oaks. The complete fortress, walls and buildings together, is reckoned to have consumed five thousand. That is a forest, felled and shaped to a schedule, in a country where oak was already the most valuable material anyone had.

Dendrochronology gives the ring fortresses a startlingly narrow window: around the year 980, in the reign of Harald Bluetooth and perhaps his son Sweyn Forkbeard. Aggersborg went up in one or two building seasons. Then — nothing. No repairs, no rebuilding, no sign that anyone maintained the ramparts. The archaeological consensus is that the fortress was in use for somewhere between five and twenty years, and hardly more. An old theory held that these were barracks for the armies that conquered England in the early 1000s, but the tree-ring dates arrived too early for that; the fortresses were already going quiet before the English campaigns began. What remains more persuasive is position. Aggersborg watches one of the three ancient crossings of the Limfjord on the north–south military road, with east–west water traffic passing beneath it. A strongpoint at a choke point, and an outpost of a state that was still inventing itself.

The National Museum of Denmark dug here between 1945 and 1954, with further trenches in 1970 and 1990. Together the excavations covered about 13,000 square metres, recorded some 19,500 individual features, and recovered more than 30,000 artefacts along with a great deal of animal bone. Among the ordinary pottery and ironwork came things that had travelled: beads of rock crystal, fragments of glass vessels, imports from Norway, western Europe and the British Isles. The most spectacular find is a damaged gold arm ring, described in Denmark as the country's greatest gold discovery after the Golden Horns; a replica is shown in the Aggersborg museum. The buried village produced its own inventory — household pots, tools, weapons, jewellery, coins — and almost no evidence of specialised craft. After the fortress emptied, nobody moved back onto the site.

What visitors walk today is partly a reconstruction, raised in the 1990s and deliberately lower than the original rampart, so the grass ring reads as an outline rather than a rebuilt wall. It works. From the ground the circle is enormous and slightly hard to believe; from the air it is unmistakable. In 2023, Aggersborg was inscribed on the UNESCO World Heritage List alongside Trelleborg, Fyrkat, Nonnebakken and Borgring as the Viking-Age Ring Fortresses — a thousand years after the people who built them stopped caring about them. The old Løgstør Municipality put the fortress on its shield. It is one of the largest archaeological sites in Denmark, and it was occupied for less time than a modern mortgage.