I’ve read great things about this castle on this forum, and just wanted to add my praise. We visited today and took the English tour (11€), which is offered twice a day (currently 1 and 4 pm). The only way to see the castle is on a tour. Our group had about 30 people and lasted about 50 minutes. It was very, very informative. Our guide, Marie, was great. We saw many different areas: fortifications, interior living spaces including the kitchen, master living/bedroom/dining areas, forge, armor room, and a display of suits of armor through history. The castle itself has a fascinating history.
For reference, we have been to Burg Eltz, which is more popular. We took that tour about 5 years ago. We like the look and setting of Burg Eltz better, but Marksburg is definitely worth a visit if it fits your schedule better. We enjoyed the tour and the history much more. I think you get a better feel for what life was like in an actual castle/fortress at Marksburg. Lunch at the restaurant was very good (and reasonably priced) as well. The gift shop has many unique items.
For those driving, as you turn right off the main road just outside of town, you will drive for about 600 meters on a narrow, mostly straight stretch of road. There are little pull outs (on the mountain side) where you can let tour busses pass you, regardless of which direction you are going). We were in an 8 passenger van. If we can do it, anyone can! There is a large parking lot near the castle (3€ cash only, paid to the attendant who will direct you to a spot). When we arrived at around 12:20, there were plenty of spaces available. From the parking lot you can walk up two steep flights of stairs, or walk up a road in one of two directions to get to the castle.
We drove from Bacharach north along the Rhine (beautiful), then across at Koblenz and down to Braubach in about 55 minutes.