If you examine a military you will find volumes of plans for incredibly unlikely situations. Once you have addressed all the likely and significant threats, you don't just stop planning--at least not any good military.
Saying that castle sieges didn't tend to involve stairway fights doesn't imply that stairways wouldn't have had defensive measures built in. That is post-hoc rationalization.
Saying that castle sieges didn't tend to involve stairway fights doesn't imply that stairways wouldn't have had defensive measures built in. That is post-hoc rationalization.