The Danevirke is not one rampart but a system, running roughly thirty kilometres across the narrowest part of the Cimbrian peninsula from the Baltic inlet of the Schlei to the marshes of the west coast, sealing off Jutland from the continent. The Hauptwall, the main rampart, runs five and a half kilometres; the Krummwall extends another seven and a half west to Hollingstedt; the Nordwall was originally 1.6 kilometres long, of which about 700 metres survive; the Kograben cuts six and a half kilometres with a V-shaped ditch in front of it; the Osterwall closes off the ground between the Schlei and the Windebyer Noor. Roughly eighty per cent of the rampart lines are still traceable in the landscape, which makes this the largest archaeological monument in northern Europe. What looks like a single decision was a thousand years of argument with geography, each generation adding to what the last had left.

For centuries the origin story came from the Frankish annals: in 808 King Godfred of the Danes, watching Charlemagne's empire swallow Frisia and then Old Saxony between 772 and 804, ordered a wall built across his southern frontier. It is a good story and it is roughly seventy years too late. Radiocarbon dating puts the first earthen ramparts at around 450 to 500 AD, and dendrochronology fixes the great timber-fronted palisade phase precisely at 737 - some seventy years before Godfred's reign. Later phases can be dated almost as sharply: the connecting wall to the trading town of Hedeby to 964 or 965, the Kograben to shortly after 983, in Harald Bluetooth's reign. Then, in the 1160s, King Valdemar the Great faced the wall with brick four to five metres high along some four kilometres - the Valdemarsmuren, still called that today - and used the whole structure as the assembly point for Danish campaigns across the Baltic.

Legend gave the credit to a woman. Thyra, wife of Gorm the Old, the first historically recognised king of Denmark, was said to have ordered the Danevirke built, and the story stuck hard enough to survive every excavation that contradicted it. That mattered in the nineteenth century, when Denmark and the German states fought over the territory the Danes call Sønderjylland and the Germans call Schleswig. The Danevirke became the argument made visible: proof that a Danish nation had drawn this line for itself in the deep past. First it served as a cultural barrier, then as a literal one - the decayed ramparts were rebuilt, entrenched and fitted with cannon in 1850, and reinforced again in 1861. When the First Schleswig War ended with peace signed on 8 May 1852 the wall had seen fighting nearby but no battle on it. Twelve years later, at the moment it was supposed to matter, it saw none at all.

The Danevirke's closest brush with destruction came in 1944. After the Normandy landings the Wehrmacht convinced itself a second Allied invasion might come through Denmark, and proposed converting the ancient earthwork into a modern anti-tank trench - which would have obliterated it. The Danish archaeologist Søren Telling heard about the plan and reached for the only lever available in that regime: all archaeology in occupied Europe ultimately fell under Heinrich Himmler. Telling telephoned the SS's Ahnenerbe office and then Himmler himself, and argued that the Danevirke was a monument of Aryan civilisation. Himmler agreed and authorised him to stop the work, warning that the written order would take days to arrive. Telling drove straight to the site and ordered the Wehrmacht officers to halt. When the local commander refused, he threatened him with the SS. Digging stopped, and Himmler's order arrived two days later. Telling later settled beside the wall and regarded himself as its keeper until his death in 1968.

The wall keeps producing surprises. In 2010 archaeologists found a six-metre gap in the fieldstone rampart near the old Ochsenweg and identified it as the Danevirke's only known gate, very likely the Wiglesdor named in the ninth-century sources. Every army, merchant and pilgrim moving between Scandinavia and the continent had to pass through that one opening. The Danevirke Museum at Dannewerk, opened in 1990, sits on the line of the rampart, and on 30 June 2018 UNESCO inscribed the whole complex together with Hedeby as a World Heritage Site: an archaeological border landscape, listed for what it says about how trade routes were controlled and defended in the Viking Age. For the Danish minority still living in this German district, the grass ridge running away across the fields is not an abstraction. It is the place a border used to be.