Sometimes history is found not with the spade but with the computer. For a long time four of Harald Bluetooth's famous ring fortresses were known. That a fifth had to exist was first revealed by a pattern on the map – and modern measuring technology laid it bare: Borgring near Køge on Zealand. A circle that slumbered a thousand years in the ground.
The Danish archaeologist Søren Sindbæk was working on a large book about the Aggersborg ring fortress when a pattern struck him: Harald's Trelleborg fortresses each lie about a day's journey – roughly fifty kilometres – apart. Yet in eastern Zealand there was a gap. There, by pure calculation, a fortress had to be missing.
A ring fortress was missing in eastern Zealand – exactly a day's journey from Trelleborg. Research approach to the discovery of Borgring (Sindbæk et al., 2014)
In 2014 researchers of the Danish Castle Centre, Aarhus University and, with the help of Helen Goodchild from York, examined an old, long-ploughed-over rampart at Lellinge near Køge. With geophysical surveys and laser scans of the terrain they made a perfect circle visible in the ground – and then, exactly where they were expected, the gates of a Trelleborg fortress. Only afterwards was the spade set to work. The site measures about 145 metres across, making it one of the largest Danish ring fortresses.
What the excavation found made the matter exciting: burnt remains of the gates. In the name-giving fortress of Trelleborg near Slagelse, arrowheads had once been found inside – signs of battle. At Borgring, too, the burnt gates may tell of warlike events. The dating points to the 10th century, to the time around 970 to 980 – the reign of Harald Bluetooth, who is also regarded as the builder of the other ring fortresses. More about these sites and the king is in our pieces on Trelleborg and Thyra Danebod.
Despite the strict geometric plan, the gates of Borgring are not quite exactly aligned to the compass points – they deviate by about eleven degrees. Probably the fortress was deliberately adapted to the landscape, so that it would not appear crooked in the terrain. A lovely detail: even the methodical Vikings made compromises when the eye demanded it.
Until 2014 Borgring was only a footnote – a rampart thought to be a relic of the Roman period. Today the fortress, together with Trelleborg, Aggersborg, Fyrkat and Nonnebakken, has been a UNESCO World Heritage Site since 2023. So the circle of Harald's ring fortresses closes – found with a technology the Vikings could not even have dreamed of.
Borgring (also Vallø Borgring) is a Viking-age ring fortress near Køge on Zealand, circular, about 145 m across, with a 10–11 m wide rampart and palisade; the gates deviate about 11° from the compass points. In 2014 it was confirmed by geophysical surveys and terrain scans (Danish Castle Centre, Aarhus University, Helen Goodchild/York) as the fifth Danish Trelleborg fortress; burnt gates were found. Dating to the 10th century (around 970–980, the time of Harald Bluetooth). A UNESCO World Heritage Site since 2023 (with the four other ring fortresses). Results published, among others, in Antiquity (2017).
Borgring was long thought to be a Roman-period rampart – only in 2014 was it recognised as a Viking fortress. That Harald Bluetooth was the builder is very likely because of the similarity and dating, but not yet definitively “proven” (a precise dating was in part still outstanding). And the burnt gates attest fire, not necessarily a great battle. What is certain is the find: a genuine, geometric ring fortress of the 10th century.
Trelleborg – Harald's ring fortresses · Thyra Danebod – Denmark's mightiest woman · Ragnar's sons · The Skuldelev ships · The Golden Horns of Gallehus.