Rhine and Mosel Tour

"How may castles" you can tour in a day probably depends on your interest. Most of the "tourable" castles are in various states of ruin. Although I wouldn't quite say "if you've seen one castle ruin, you've seen them all", you might reach your tolerance level rather quickly if you go from one ruin to the next in a very short time. Most are self-guided.

Other castles, especially those that have been refurbished, operate as hotel/restaurants, or remain private residencies. Hence, you can't visit them, unless you have business there. Marksburg is one of the few non-ruins that you can visit, but you would need to time your visit carefully if you want to catch one of the infrequent English tours (you can only visit by guided tour). You would also need to carefully time a visit to Pfalzgrafenstein (that really cool looking castle in the middle of the river), which is only open when water levels on the river allow access.

PS- If you have a long trip, and you want to make castles the focus, also consider checking out the Bergstraße between Darmstadt and Heidelberg, and the Neckar River Valley between Heidelberg and Bad Wimpfen. The latter forms part of the Burgenstraße (Castle Road, like the more well-known Romantic Road).

"Can anyone give me an idea of how many castles you can tour in a day, without rushing... how much time should be allowed for a tour of the Rhine and Mosel areas?"

Most tourists zoom through the Rhine, spending just one night in Bacharach or wherever on their way to somewhere else, but there's far more than just a 2-hour river cruise and castle tours, and a week isn't out of the question at all. I hope your question doesn't mean you're considering only 1 day in the area.

Marksburg (tour) and Rheinfels (self-tour) are the most commonly visited castles. Both are great. It's easy to see both in one day if you want. Staying in St. Goar (on the west bank, where most of the Rhine towns you'll want to visit are located) makes it easiest - Rheinfels is right in town and you can see it whenever. Marksburg English tours are typically at 13:00 and 16:00. On German tours, take Rick's handout with you or get one at the castle; guides can normally answer questions in English.

Rick's handout

To get there from St. Goar, take the ferry (runs all day) across the river to St. Goarshausen and catch a northbound train (20 min. ride to Braubach.) These trains run every hour at :30 after on weekends, more frequently on weekdays. OR... do a river cruise (Bingen's a good place to start) and end up in Braubach by boat 2 hrs. 50 min. later.

On the Mosel: I would target 2 castles - Burg Eltz (near Moselkern) and Reichsburg in Cochem. Both offer guided tours only. Reichsburg also has a falconry show on castle grounds. You could do both places in one day with an early start and a late return to St. Goar or your Rhine base town. For such a daytrip, Boppard (which has a cool chairlift ride to a local clifftop) is closer than St. Goar and will make a slightly better base town for both rivers than St. Goar.

Other great outings in the Rhine Mosel region:

Bacharach
Oberwesel
Peace Museum Bridge at Remagen
Bad Sobernheim open air museum
Linz am Rhein
Cologne
Mainz
Trier
Katzenkopf (Nazi bunker, Sunday afternoons, near Trier

(The last 2 would be pretty far for daytrips, actually; consider a change of base town in/near Trier or Cochem.)

I think the best castles to see in the area are Burg Eltz (Mosel area), Marksburg (Rhine), and Rheinfels Castle ruins (Rhine). I went with my brother a few years ago and we did the following: Day 1: arrive in Frankfurt, pick up rental car, visit Rudesheim and take the chairlift up to the Neiderwald monument, take ferry across to Bingen, visit Burg Rheinstein, go to St Goar and check into hotel. Day 2: visit Marksburg castle, visit the town of Bacharach, check into Liebenstein Castle (our accommodations for 2 nights). Day 3: visit Pfalz Castle (on small island in the Rhine), visit Rheinfels Castle ruins. Day 4: drive up the Mosel river valley, visit Burg Eltz, visit the town of Cochem (we didn't bother with Cochem Castle), visit Beilstein (beautiful little town) and check into hotel. So there you have it, you can do two castles per day and also drive up and down the Rhine a few times to see other castles that you can't go to visit. We had a relaxed time and saw a lot. Three nights in the Rhine and one in the Mosel were plenty -- after that we headed off to other parts of Germany and Austria. Of course, we included a stay in Fussen so we could see the three King Ludwig castles in that area (Neuschwanstein, Hohenschwangau, Linderhof).