Altdahn
Altdahn is a fabulous castle, not because it has much to see, but because of the way it was constructed and what ruins remain.  In many ways, this castle is a dream for the young and young at heart.
Altdahn was built on several pillars of sandstone, and is essentially three separate castles linked by walkways that could have been blocked or removed had any one of them fallen to an attacker.

Each castle was built in sequence. 
The photos above and to the right show how Altdahn was built right into the living rock, enhancing its protection from attack.
The stairs to the right are obviously rebuilt as they are like new.  However, some of the stairways of the castle appear to be original to the structure.

Below, a nice view of the hills nearby.
The first castle, called Tanstein, was built in the early 1200s by the Knights of Dahn.  This castle was located on the western edge of the sandstone escarpment (the far left of the photo).  This small band realized that they needed an additional structure, and so built the center castle in 1287.   Altdahn's construction was sometime in the mid-1200s and is located on the eastern edge of the escarpment.  The central castle changed hands several times, and finally was consolidated under Count Johann von Sponheim, who renamed the central section as Grafendahn.  Interestingly enough, the two outer castles remained in the hands of the Knights of Dahn until the early 1600s when the family line died out.  The central castle went to the Counts of Palantine in 1464 and later passed to several other families.  The castle was destroyed by the French in 1689.