While we visit Germany for 2 weeks, we are meeting up with our daughter who is studying 6 weeks in Luxembourg. Meeting her in Heidelberg then on to Bacharach. We have 3-4 nights together. Thought it might be fun for her to stay in a castle hotel? Are there any between the two towns?
The only castle between the two locations that offers lodging of which I am aware is Starkenburg, which sits on a small mountain above the beautiful wine town of Heppenheim. However, I think it's strictly a youth hostel.
Now, if you don't mind something near Heidelberg but not between there and Bacharach, travel about 30 minutes up the scenic Neckar river valley to Hirschhorn. Burg Hirschhorn serves as a hotel and restaurant. Once you see the Neckar valley, you'll probably want to explore further, even though most English-language guidebooks don't mention much about it. The wine towns of the Bergstraße (the road that runs from Heidelberg to Darmstadt along the western edge of the Odenwald mountains), which includes Heppenheim, are also worth a visit.
Staying in an 800 year old castle in Bacharach:
Here's our experience staying at Burg Stahleck. We walked the thousand steps incl luggage to the Burg (just about did me in) and registered. What a busy place for a DJH (German Youth Hostel). A very popular place - you need to book in advance. A lot of travellers without reservations were turned away. We ended up in a Turmzimmer (tower room). Small, 3 bed room with its own bathroom. My son and I thought we had the room to ourselves - but at 9 pm a Japaneses student showed up. His first day in Germany. We showed him how to set up his bedding. He spoke very little English - but we laughed a lot. Dinner was buffet style with a good variety. Same with breakfast (incl in our E 24.00/ night). From the terrace you'll have an incredible view of the Rhein river. It's easy to meet travellers from all over the world. After breakfast, we exchanged small gifts with our Japanese student and made our way to the KD line ferry dock for our trip to Koblenz. Great memories staying in a 800 year old castle - but no Hilton. Membership card in HI (Hostelling International) and reservation a must. Here's their website: http://www.hihostels.com/dba/hostels-Bacharach-022077.en.htm