This is all over my feed at the moment. I'll preface it by saying that Spencer McDaniel is one of the best popular history writers in the field, and if you're not familiar with his work, you should be. But he's not perfect ...
I think he's half-wrong on this one. Right that there's no evidence for the defense hypothesis, and that the most likely reason is simple balance. Very very wrong about the idea that the attacker has an advantage fighting up a staircase.
Striking upward with a spear or a sword is exhausting, and with an axe it's nearly impossible. Whatever advantage the attacker would gain by access to the defender's ankles would be *vastly* outweighed by the defender's advantage in energy for strikes at the attacker's head, neck, and torso. There's a reason why holding the high ground was the primary tactical goal for most of recorded military history, right up until guns took over entirely—and it's *still* an advantage in most cases, because the poor bloody infantry still has to slog to the top of the hill.
I've never fought on a staircase, but I've done SCA melee combat on very steep ground. Zero doubt in my mind that it's better to be fighting from above than below.
Of course it's quite true that once an attacker was inside the castle, the defender had failed in the main goal: holding the high ground of the castle walls! But not every successful breach ended in victory. It's usually worth trying to hold whatever advantageous position you can ... compared to the alternative.
