Treis-Karden

One of the most beautiful castles in a region packed with them, founded sometime before 1000 AD and still lived in by one of the three branches of the Eltz family. Access to the interior is by guided tour only, departing every 15 minutes in German and on demand in English. The ticket also includes entry to the treasure vault, showcasing medieval weaponry and a number of impressive pieces of golden artworks and jewelry. No photography allowed.
To get to Burg Eltz, drive to the car park and walk 800 m, or hike 1½ hours directly from Karden. On public transport, head two stops east by train to Moselkern, and hike around 5 km from there, following the signs to Burg Eltz; the first 30 minutes is along paved roads through the village, the second half a gentle uphill trail along and above the Elz River. Note that the station in Moselkern is unmanned and has no storage facilities, but the restaurants along the Mosel around 500 m from the station will be happy to hold your bags if you eat your lunch there.