Eresburg, Hill castle in Marsberg, Germany

Eresburg is a castle ruins perched on a plateau overlooking the Diemel River valley below. The site displays layers of fortifications and settlement traces spanning multiple historical periods.

The fortress fell to Charlemagne in 772 during the Saxon Wars and was quickly transformed into a Frankish military base. This victory shifted control of the region and forced the integration of diverse peoples under new rule.

The Irminsul, a great Saxon pagan sanctuary, stood here or nearby; Charlemagne had it destroyed in 772. He charged Abbot Sturmius of Fulda with missionary work in 779 and had a church built, probably on the former temple site, where he wintered in 784/785. A crowned head on a pillar in the Obermarsberg collegiate church recalls Thankmar's murder in 938.

The hilltop is reachable on foot and offers broad views across the surrounding landscape from above. The ground is uneven and climbing requires some effort, so good footwear and time for exploration are worthwhile.

A crowned head carved on a pillar in the former Obermarsberg collegiate church recalls Thankmar, half-brother of Otto I. Besieged at the Eresburg in July 938, he surrendered in the church and was stabbed in the back by a retainer of his other half-brother Henry.