Rheinfels Castle above the town of St. Goar is the most substantial castle ruin on the Middle Rhine — a vast warren of walls, towers, tunnels, and outworks spread across the hillside above the river. It is not intact like Marksburg, but its scale is impressive precisely because of its incompleteness: you can wander through roofless halls and along surviving battlements with views down the full width of the Rhine.
The tunnel system is particularly atmospheric — bring a torch (flashlights are available to borrow at the entrance) and work through the passages that were used for storing supplies and moving troops unseen. The history exhibition is good and explains the castle's rise from a toll castle to a military headquarters during the Thirty Years War. A hotel inside the ruins offers the most theatrical overnight stay in the valley.