Rheinstein / Reichenstein
Rheinstein is an awesome caslte.

I could simply leave it at that.  It is worth a visit.  Unfortunately, on my visit in 2005 it was adorned with scaffolding, and thus the BEST photo of the castle (see below) was spoiled at that time.  However, I got a good view of it in 2007.  This castle is so impressive the way it is perched on a rock ledge along the river.

Rheinstein is across the river from the town of Assmannshausen, which is also worth a visit.  I did this by hiking from the Niederwald-Denkmal, a walk that took me about three hours round trip.  Just follow the small roads that lace the vine-strewn eastern slopes of the river.
To the right is a view of Rheinstein from the other side of the river.  It was originally built around 900 AD as a customs collection point, and the Archbishop of Mainz expanded it into a fortress.  It eventually fell into ruin from neglect, but was restored by Prince Friedrich Wilhelm of Prussia in the 1830s.  It was finally purchased by the Hecher family in the early 1970s, and has undergone extensive interior restoration since.
Two views of Rheinstein from the highway below (left 1979, right 2005).  If you look carefully at the left tower in the right photo, you can see a person ascending the stairs.  To reach the castle, there is a small (VERY small) parking area along the highway below.  A short walk up, and you're there.
Possibly the BEST angle shot of Rheinstein, and perhaps any other castle in Germany.  I was hiking along the slopes back in early 1979, turned the corner, and there it was.  To see it in person is outright breathtaking.  The photo below was taken from the same position in 2007.

You can still get to this view from the footpath below.  Simply take the trail that takes you away from the castle until you reach a small outpost building.

Here is the official website for the Rheinstein castle.