The Eresburg is the largest, well-known (Old) Saxon refuge castle (Volksburg) and was located in the area of the present German village of Obermarsberg in the borough of Marsberg in the county of Hochsauerlandkreis.

[1][2] Earlier research viewed the Eresburg as a border castle of the Cherusci tribe and a fortress of Prince Segestes, who was supposed to have held his daughter, Thusnelda, prisoner here.

Thanks to its favourable position on the border, the castle was repeatedly attacked and hard fought for before being conquered in 772 A.D. during the Saxon Wars by the Frankish king, Charlemagne.

On the one hand, therefore, Charlemagne captured the Eresburg and, on the other, "pervenit ad locum, qui dicitur Ermensul", i. e. he found the site that was called Irminsul.

The third part of the sentence runs "et succendit ea loca", i. e. he set "these places" (plural) on fire,[5] possibly implying that Charlemagne moved on from the Eresburg to the Irminsul.