Marksburg was built around 1100 to protect the town of Braubach. Under Napoleon it was used as a prison and in 1900 it became the first castle of the newly established German Castle Association. It is now part of the Unesco-protected Rhine Gorge, which I did by boat and which I recommend.
The rooms show late mediaeval furniture and furnishings. It was here that I learnt that window comes from 'wind eye': a hole in the wall to let light through (and for castles like this one, exposed on a hill, a lot of wind as well). Apparently in winter the holes were blocked with linen dipped in fat.
If you want to go, wear proper shoes. The road up there is steep and irregular and no cars are allowed: www.marksburg.de.