Ehrenfels and the Mausturm
Ehrenfels is one of my favorite castles, not because it is the most elaborate, nor the largest. In fact, you can't even get inside of it!  Rather, it is generally ignored by the army of tourists who flock the Rhein.  Thus, it is quiet, even serene, and allows one to simply contemplate what the Rhein looked like 400 years ago. 

Years ago one could get inside of it, and I even contemplated trying to climb up into the south tower (left) which dominates the curtain wall.  But descretion was the better part of valor, for Ehrenfels is slowly falling apart.  For this reason it is now closed, a metal gate blocking the main entrance.  You might find a way in, but I DON'T recommend it.

Ehrenfels was once much larger, and extended all the way down to the river.  Like all Rhein castles, its primary mission was to serve as a toll station along the river, as the Rhein was one of the main arteries of trade through central Europe.  While Rhein castle owners have been called "Robber Barons" in many sources, this is actually an unfair characterization, for many of the Rhein castles provided security for the region that allowed trade to flourish.  Naturally, they expected to be compensated for their efforts, just like governments do today.
The photo at the right offers a glimpse as to the extent of the southside fortification.  The foundations for several rooms can be seen, as well as the fireplace halfway up the wall, where a wooden floor had once been.  As of 2007, even these outer building foundations are blocked off by a grated fence.

One of the best ways to get to Ehrenfels is to simply walk from the Niederwald-Denkmal, since your parking there is for the entire day.  The walk takes about a half an hour, perhaps a little longer.  There are no signs... instead, follow your instincts and you will arrive above the castle (above). 
Left, an awesome profile view of Ehrenfels showing the height of the towers and curtain wall that protected the ward from any attacks from above along the slope.  The steepness of the slope can also be clearly seen, being about 40 degrees, or 90%.

This shot was taken in 2007 from one of the boats on the river.

Below, Ehrenfels and the Mausturm seen in March 1945 from Bingen. This photo comes from the US Army's "Pictorial Record- The War Against Germany and Adjacent Areas."