The areas north of the Elbe were at first given to the Obotrites, while Land Hadeln was directly incorporated. However, Nordalbingia soon was invaded by the Danes and only the intervention of Charlemagne's son Charles the Younger in 808 pushed them back across the Eider River. The next year the emperor had Esesfeld Castle erected near present-day Itzehoe and the entire region was incorporated into the Frankish Empire. In order to encounter the ongoing invasions led by King Sigfred's successor Gudfred, the Franks probably established a Danish march stretching from the Eider River to the Danevirke fortifications in the north. After King Gudfred was killed, his successor Hemming concluded the Treaty of Heiligen with Charlemagne in 811, whereafter the Eider should mark the border between Denmark and Francia. However, quarrels between both sides would continue for more than a century until the East Frankish king Henry the Fowler finally defeated the Danish forces of King Gnupa at Hedeby in 934.