King Henry the Fowler in 929 marched against the Polabian Slavs settling east of the Elbe River and defeated them in a battle near Lenzen. Occupying the eastern riverbank, Henry had a fortification built on a hill above the Havel tributary, near its mouth into the Elbe. His son Otto I continued the expeditions and in 936/37 established the Saxon Eastern March ("Marca Geronis") on the conquered territories. In 948 he founded the dioceses of Havelberg and Brandenburg, initially suffragans to the Archbishops of Mainz, from 968 to the newly established Archdiocese of Magdeburg. Part of the Northern March from 968, Havelberg diocese was occupied by revolting Lutici tribes in the Great Slav Rising of 983 and merely remained a titular see.