Castle Marksburg is the only castle on the middle Rhine which was never destroyed or conquered, and is a good example of how medieval architecture evolved over the centuries. You can spot the castle from miles away as you're coming up the river. To actually get there, you must first arrive at Braubach and hike up a trail for about 10 or 15 minutes.
The visits are guided (in English, German, or Italian), but there are also pamphlets in other languages. Once you enter, you pass through two massive doors and arrive in a courtyard displaying the coats of arms of the families that lived there. The entrance itself is a sort of ramp carved into the rock so that the knights could enter on horseback.