Schauenburg
One of the problems of castle hunting is that sometimes you come across a castle where there is simply not much left.  Schauenburg is essentially one of these, though I've seen ones in worse shape than this!  Schauenburg is just north of Heidelberg, and is little more than the remains of a ringwall.  The main wall is about six feet thick, which is pretty good for a per-cannon era fortress.  These photos were taken about 1978.