The Danube and Rhine Rivers are the two largest rivers of Central and Western Europe. That is if the rivers of Eastern Europe are not taken into account. These two rivers were essentially the borders of the Roman Empire.
Map 1 Rhine and Danube
Map 2 The Roman Empire
The Rhine and the Danube formed most of the northern inland frontier of the Roman Empire and, since those days, the Rhine has been a vital and navigable waterway carrying trade and goods deep inland. The many castles and fortifications along the Rhine testify to its importance as a waterway in the Holy Roman Empire. In the modern era, it has become a symbol of German nationalism.