Germany in March and what is open

I would be happy to explore the smaller quaint towns while they are a ghost town!

I have been to several of the places you mention, once in the fall and once in summer. All of these cities/towns will be “open”, just as your hometown is “open” in March. You will want to check the hours for specific sites (castles, museums, etc) you want to visit, as you are traveling off season and some hours may be more limited. You may find some smaller shops/restaurants close between peak season.

The opening hours in the places you mentioned will all be normal, although some places still have the slightly shortened winter hours. Even the Franconian Open Air Museum in Bad Windsheim, a recommended day trip from Nuremberg, will return to its normal summer schedule on March 5. The only restriction I can think of concerns boat excursions on the Rhine, where the summer schedule does not usually begin until Easter (e.g. the trips of the historic paddle wheel steamer "Goethe" from Rüdesheim to Koblenz). But shorter routes, such as Boppard-St. Goar, are also served in early spring.
In any case, be prepared for cool weather and bring rain gear.

March will still be Winter weather in Germany. Still, German Winters are long, but not severe. Still, it will be cold, take a very warm overcoat, warm shoes, gloves and a warm hat.

Enjoy some Gluwine.

If you go toward the end of March, there's a chance the Winter weather will have changed over to Spring. We've been in Germany & Austria about that time, and the thaw appears quickly.

Not having crowds would be a welcome sight. I was worried restaurants and shops would be closed for the winter. When I was stationed in Italy, I went to Positano during off-season (march) and a lot of the shop owners had not returned yet and many shops were closed. I wasnt sure if that would be the same in Germany.

We usually visit in early April.

Nürnberg is a major city, things will be open. A different concern might be trade fairs. If one is in town the rooms tend to fill and prices go up. Just use a hotel booking website to check for rooms before planning your itinerary.

Bamberg, Würzburg and Regensburg are college towns with plenty of historic sights. They will be open and lively.

Rothenburg is up and running in April after a winter slowdown. The spring equinox is March 20 so the very short days of winter will be gone. I like Rothenburg best for walking about. It is fine in spring.

We found the Middle Rhine very quiet in early spring. I might be tempted to stay in Mainz and day-trip by train to St. Goar, Bacharach, etc. Some folks really like Boppard as it has a larger selection of hotels and restaurants.

I would not be reluctant to visit in March. Take layers of clothes as temperatures could vary a lot.

Contrary to popular belief, Germans do not hibernate. Life goes on, albeit a little more indoors. I've been there in the months of December, January, and March, and, except for touristy venues, places that are often closed in the winter for lack of tourists, things were pretty normal. I was in Bacharach one November, and a lot of the hotels were closed for the winter (again, due to lack of tourism business that time of year).

Kaiserburg in Nürnberg is open in March with winter hours shorter than in the summer.

In Rothenburg odT, the Nightwatchman Tour will not be running. Looks like the Christmas Shop (isn't that why everyone goes there?) will be open.

I don't know what would be "closed" in Bamberg. They don't take down the Rathaus on the bridge, which was what I went there to see.

Things on the Rhein are less open. Rheinfels castle says they are open (except for the museum), but close in adverse weather for safety reasons. Burg Eltz (Mosel) is closed. The Marksburg specificly says that the English language tours are not given in the winter, but it's a little vague about whether there are German language tours.

The Rhein day cruise ships, of course, aren't running. I was there, in Boppard, at the very end of March, in 2004, and had to wait until the first weekend in April to take a day cruise. I was thinking of cruising to Braubach on my first day, Friday, to take a tour of the Marksburg, but the ships weren't running yet. I crossed the river on the car ferry, walked into Filsen, and took the train to Braubach.

You can still visit the Rhine in March. One major sight on the Rhine is Marksburg Castle, a never-destroyed, completely-intact castle from the Middle Ages with a true Medieval feel - and it is open year-round. It's located in the town of Braubach, near Koblenz.

https://www.marksburg.de/en/circuit/#/

Guides there normally speak English and can answer questions for those who do not speak German. Our language is a very useful tool for Marksburg guides responding to questions from visitors around the planet; it's not just the native English speakers who will ask them questions, but like most Americans, other European visitors (Dutch, French, etc.) often lack German language skills, and just like you, they will ask their questions in English. Here's a written text of the tour in English:

https://www.marksburg.de/assets/uploads/Marksburg__Guided_Tour_in_Englisch_2021_.pdf

Most visitors in March will want no part of a river cruise or a chairlift ride (like Boppard's) because of the cold weather. But you CAN use the Koblenz Cable-Car to reach Ehrenbreitstein Fortress - it should be open on weekends in March, as it was in 2022:

https://www.seilbahn-koblenz.de/homepage.html

The town of Rüdesheim is probably the liveliest of the small Rhine Valley towns in March. This gateway town to the Rhine Gorge gets more visitor traffic than the others at that time of year. You might want to book there for a wider choice of restaurants and hotels. Rüdesheim could serve as a good base town for outings in March. You could day trip south to Wiesbaden (30 minutes by direct train) and Mainz (1 hr.) by train. Rüdesheim is also on the same train line as Koblenz (1hr., direct) and Marksburg Castle (in the town of Braubach, 45 min's., direct.) If in fact you ARE brave enough to take a wintertime cruise, Rüdesheim is the place to board; cruises are "täglich auf Anfrage", meaning daily but subject to advance reservation... and then only if a sufficient number of visitors signs up. The current schedule offers a departure from R'heim at 10:00 am - find it on the following page in the bottom right corner

https://www.k-d.com/fileadmin/KD_Fahrplaene/KD_Winterprogramm_FRA_Mainz_2022_web.pdf

So if you want a small atmospheric Rhine town, I think R'heim is a very good bet.

If you want a city with city stuff, and lots to offer on its own," then MAINZ, which also offers a quick, direct shot at FRA airport, is a good choice. But Mainz is well-separated from the scenic part of the Rhine, something to keep in mind.

If you want a city that also offers rail access to the MOSEL River Valley and its towns (Cochem, Trier, others) then KOBLENZ is where I'd suggest for a March base. The chairlift and fortress are right in town, and Marksburg (Braubach) is just 10 minutes away by train. Trier (Roman history, 1.5 hrs. by direct train, and open for business) and Cochem (45 min's. by direct train) are both worth a visit and easy to reach. Rüdesheim would be reachable as well, of course, and while the west-bank Rhine towns of Bacharach and Oberwesel will be extremely quiet, you could still catch a direct train there from Koblenz just to check out all the fine old-world buildings.

Wherever you book, arrange to visit the Rhine on Saturday and Sunday for maximum access to sights, restaurants, etc.