Yeah, the disability is tough. I have problems with my hip, so I do try to be careful with not doing too many stairs, which can be a problem because DS is terrified of Elevators. Most places now are required to have access for disabled people, but not all the historic places can do that. With Neuschwanstein, there's a steep hike up the side of the mountain....45 minutes, but they DO offer carriage rides, and I'm sure they have an option for disabled people, but I remember there were also a number of stairs in the castle itself, so I don't know how they handle that aspect of the tours.
And I bet it depends on the country. Most of Europe is pretty good about that, but I don't know about some of those countries in Asia, or Africa, where it's not as developed. My best friend is in Thailand right now, and heading to Malaysia in a few days, and she's been to Vietnam, Cambodia, etc. I'll have to ask her what accessibility is like.