Poul Nørlund excavated the site from 1934 to 1942 and published in 1948. The plan that emerged is almost aggressively geometric: a circular rampart of turf and timber, 19 metres thick, enclosing an interior 136 metres across. Two roads crossed at right angles in the exact geometric centre, running out to four gates set opposite one another at the cardinal points. Each of the four quarters held four longhouses arranged in a square - sixteen in all, each around 29 metres long, bow-sided so that the walls curved like the hull of a ship turned upside down. Nothing about it is improvised. Somebody surveyed this ground with instruments and a unit of measure and expected the builders to hit the marks, and they did.

Dendrochronology settled the date with a precision most Viking-age sites never get: the timber was felled in the autumn of 980 and put up, presumably, the following spring. The construction was fast, and then it stopped. There is no sign of maintenance anywhere in the structure, which suggests the fortress was in use for no more than ten or fifteen years. Parts of it were never finished at all - the outer eastern rampart appears to have been abandoned mid-build. The finds record an attack: dead soldiers, traces of burning, a suggestion that fire ended the place. Only the ground around the gates shows the wear of long use. Trelleborg is attributed to Harald Bluetooth, whose reign ended in the mid-980s, and it looks less like a settlement than like a project cut short with its patron.

Five ring fortresses are now recognised in Denmark - Trelleborg, Fyrkat, Aggersborg, Nonnebakken and Borgring - and Trelleborg is the one archaeology knows best, the best preserved and among the few excavated in full. It is also the odd one out. Alone among them it has an outer bailey beyond the ring, holding a further fourteen longhouses, and alone among them it has a cemetery attached to that bailey. Nørlund's team excavated 135 graves. The whole complex may have housed something like 1,300 people. And the site's own name, Trelleborg, became the Danish word for the entire class: the others are trelleborge because of this one. The setting explains the choice. The fortress stood on a peninsula between two rivers, jutting into swamp that connected through a lake to the Great Belt - shallow, awkward country for an army on foot, and navigable water for anyone arriving by ship.

Strontium analysis of the skeletal remains produced the most human result the site has yielded. A large proportion of the dead did not grow up on Zealand. Their bones point to what is now Norway and what is now Poland, which suggests a garrison of foreign fighters stationed here as castle guards rather than a local levy. The Polish connection fits what is known of Harald Bluetooth's politics: he was closely allied with the Obotrite Slavs through his marriage to Tove, daughter of the prince Mstivoj. It fits his end, too. Harald fled south to Wolin, known then as Jumne, and died there in 986 or 987 of wounds taken fighting off the men pursuing him. The soldiers buried outside the rampart at Trelleborg had come a long way to serve him, and most of them did not go home.

A full-size longhouse was reconstructed on the site in the 1940s; later research has questioned the details of how it was built, which is its own kind of honesty about what excavation can and cannot tell you. The Trelleborg Museum opened in 1995 and holds part of the finds, with the rest in the Nationalmuseet in Copenhagen - the historian Kåre Johannessen, later of Middelaldercentret on Lolland, was its curator from 1999 to 2004. In 2023 Trelleborg was inscribed on the UNESCO World Heritage List together with Aggersborg, Fyrkat, Nonnebakken and Borgring, as the Viking-Age Ring Fortresses. Slagelse Municipality has committed 25 million kroner to a project called Ny Trelleborg, bringing digital and virtual reconstruction to the museum, and a wider restoration is under way across the whole Tude Å valley. Michael Crichton put the fortress in Eaters of the Dead as 'Trelburg', and confirmed in his footnotes that he meant this one.