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. It was a hill castle built on the plateau of a low table hill, known as the Eresberg, at a height of 130–150 metres above the Diemel, a tributary of the Weser, in the extreme south of the Saxon Gau of Engern on the border with the Duchy of Franconia.