By car, I drove on the left side of the Rhine from Koblenz to Bingen. There I took a ferry and drove the same 55 km back. This is a really great way to enjoy all the wonderful views in this area.

The funny thing is that you get the best views of the monuments on the opposite side of the Rhine. So when you see a really nice castle, the Rhine is always between the castle and yourself.

However, I did enter some of the most interesting ones. The Marksburg Castle is surely not to be missed. It still is quite intact, and there are guided tours that take you into the Middle Ages (at least for an hour or so).