Ehrenbreitstein
One of the more awesome views along the Rhein is that of the Deutsches Eck as seen from Ehrenbreitstein. This is the confluence of the Rhein and Mosel Rivers, and was so named by the Deutschordensritter (German Order of Knights) in the early 1200s.  In the Koblenz section there is more on this unique feature.

Views of the Rhein and Koblenz are excellent.
Ehrenbreitstein is a maze of corridors, tunnels and walls.  It was designed as multiple fortressess within a fortress, and any besieger would have to spend inordinate amount of time to reduce each part.

As one would work their way into the fortress they would be confronted by these corridors with countless embrasures for early firearms.

Below, the facade of one of the interior structures facing the parade ground, which is absolutely huge.  This facade is in a Roman architectural style as a remembrance of the greatness of the late Roman Empire.
Below, additional views of these corridors.  Any effort to scale these walls, or even bring artillery within to blast them, would be for naught.

In the photo at lower left, you can see the embrasures at the base of the wall to sweep the corridore with musket and light artillery fire, the latter using cannister or grape shot.  The photo at lower right shows multiple embrasures, many now sealed shut, to employ artillery and muskets.