Castle Hotel between Strasburg France and Frankfurt Germany

There are several along the Rhine downriver from Frankfurt like that, actually northwest of Frankfurt. Not really between Strasbourg and Frankfurt, where the Rhine is pretty flat country. Might be something along the Neckar, southeast of Frankfurt.

We stayed in a nice Schloss Hotel along the Nekar River (not too far from Heidelberg) called Hirschhorn. It had a fantastic restaurant. We had a nice room and it was fun to say we stayed in a castle. The outdoor terrace had a great view of the river and the locks.

I stayed in a really cute baby castle on the Mosel, in Zell. It was great, but it is not on the route you've specified. Here's a website showing many castle hotels in Germany, have you already seen it? Schlosshotels

Love the schloss in Hirschhorn! I haven't stayed there, but we relaxed on that scenic terrace with a beverage and had a fantastic modern German dinner in town at Das Hirschorner Kartoffelhaus. Outside of Heidelberg, that section of the Neckar river is largely unknown to foreign tourists, despite being very scenic and being wine region. I'd absolutely recommend it.

We stayed at Castle Hotel Auf Schoenberg a few years ago. It was unforgettably special albeit quite a $plurge.
We were able to select the room we wanted. We opted for an apartment- sized room with a small outdoor terrace and a bathroom you entered through a false bookcase door. The most special part were the nightly multi- course dinners. You were given a small bound book ( to keep as a souvenir) with the menu selections for that evening. Each night was different and every course was delicious.
The only downside is that the castle is located on the top of a steep hill. While that provides spectacular views, it also challenges you if you are without a car as we were. We took the train from
Frankfurt to the Oberwesel station. There we called the local taxi ( a great guy we used several times). He transported us up the hill. We would walk down each day to begin our adventures and then have him return us in time for dinner. Oberwesel is a charming little town. We also did a boat ride down the Rhine using the Rick Steve’s audio tour. At the end of our 3 night stay we took the train from Oberwesel to Frankfurt where we caught a train into Paris.
It was a truly memorable stay.

From what I've read, the hotel at Schloss Hirschhorn is closed for repair/renovations. Please let me know if this is not true.

The castle hotel at Auf Schoenburg looks wonderful!

Well shoot, it was a lovely place. Hopefully they will re-open it after renovations.

TravelingMom, what month did you stay at Auf Schoenberg Castle Hotel? We will be in that area during second week in December and will have a car. Just a little concerned about the weather. Departing from Frankfurt on Dec 15.