Castles, castles and more castles

On most of the rivers between Amsterdam and Koblenz the shoreline was about the same elevation as the river. That changed at Koblenz where the soil changed  to granite. Rather than be a plain, things changed to a canyon cutting through the mountains.

In medevil times in such a canyon a canon could control passage of a ship. So the guys with money did. They built castles to house the canons and in which to live. For many kilometres along the Rhine it seems that someone built a toll booth, called a castle today, at every bend.