Even if you can prove it true in some specific castle that doesn't not mean it is true for all. Maybe other castles just copied the one where it was - often type of thing is done in architecture without realizing why, and so one castle built for fighting on stairs was copied in others without the other features that make fighting on stairs possible. (Or maybe the first was built for fighting on stairs, but after it was built practice proved it was still a bad idea - lots of variations on this idea).
