Marksburg Castle on the Rhine
The Rhine Valley is one of the most castle-studded areas of the world. There are countless castles along this romantic river. The whole area is easily accessible and can be seen in a variety of ways, from trains, riverboat cruises, or by car. It is also a relatively "compact" area that can be covered pretty thoroughly in two days and reasonably well in one day. It's a great area to stay in if you want to make day trips to either the nearby Mosel valley or the cathedral city of Koln. Boat cruises are popular and cheap. You can also follow the river by train making stops wherever you want. Towns also have cathedrals to view. It is one of the most beautiful areas in all of Europe, though there are plenty of other tourists to contend with.