 To reinforce their conquests, the Germans quickly built up an extensive system of fortifications between the Saale and the Elbe, including a fortress at Meissen, facing little resistance from the Sorbs.By comparison, when Henry I attempted to impose tribute upon the Veleti, the result was war.In late July or early August 929 the Redarii, a German tributary and a sub-group of the northern Veleti, rebelled and captured by storm the strategically important German fortress of Walsleben, massacring the garrison and the civilian population.