(The abbey was also exempt in Rouen, Amiens, Dorestad, and Maastricht.) Charlemagne wanted to have better control of this taxation process so he appointed abbots to fill this role.
In the Viking period the route Eider - Treene - Rheider Au - Schlei served as a navigation way and/or transport or trade route between places to the north and the Baltic Sea, as commercial centres functioned (see Dorestad, Haithabu).