During the highly successful Rhine Raids, the tide against the Frankish Empire was turned. Byzantine ships forced the retreat of the Vikings from the Rhine River. Many ships were captured and many more were destroyed at the hands of the Byzantines due to their utilization of Greek fire. During this time, the Franks worked on holding the Danes back. When the Rhine Raids died down, the Byzantines and Franks were able to both focus on the marauding Danes. The lines were pushed back to Hammaburg Castle, a fortress constructed by Charlemagne during the Saxon Wars.