This is one of the best values I've had with an AirBNB or VRBO, and we used these services in a dozen countries. As a value for what you get, it's hard to beat.
Yes, there are stairs, but the place is terrific. If stairs are a problem it would be a challenge, but we actually thought it was cool. It's a remodeled apartment complete with stone interior walls, windows that look out on the nearby church and cathedral, and a location that's literally only steps away from Edinburgh castle.
I stayed here with my wife and two sons, ages 16 and 19. We had plenty of room, and appreciated having a washing machine midway through our trip. The building is right on the Royal Mile, and it's on the high end. So, other than a 200 yard walk to Edinburgh castle, it's all downhill from here. It's clean, quiet, and in a building that appears to be authentically old.
I'd definitely stay here again if I came to Edinburgh.