Alzenau
For a long time I could not identify this castle.  I searched maps and photos and eventually searched on Google Earth.  Finally... at last!  I discovered it almost by accident.

This is Burg Alzenau, taken around 1978 or 79.  At the time the castle was in poor condition.  Today, the castle has had restoration work and the trees blocking the view have been cleared away.

I passed the castle while enroute to search for three other ruins further east.  I finally found one of the ruins after two attempts... and all that was left was a pile of rocks!