Rhine
The Rhine is a river that flows from Grisons in the eastern Swiss Alps to the North Sea coast in the Netherlands and is one of the longest and most important rivers in Europe. 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.

It has also served as a defensive feature and has been the basis for regional and international borders. The many castles and prehistoric fortifications along the Rhine testify to its importance as a waterway. River traffic could be stopped at these locations, usually for the purpose of collecting tolls, by the state that controlled that portion of the river.
Rock River Star