How is that a rebuttal? You plan your castle's military features (assuming you do plan them, which sort of begs the question) with the majority of attackers/defenders in mind.

Or do you mean "if the mason who planned the castle was left-handed, he would have built anti-clockwise stairs"?

I feel like right-handed people would prefer anti-clockwise stairs though, because if you're carrying something heavy it's usually harder to climb up than down, and in my experience is more comfortable to carry a bulky object on the outside of the spiral case than on the inside.

And as far as I know right-handed people have always been more common than lefties, so having more clockwise than anti-clockwise staircases is counterintuitive to me.

The best rebuttal would be if someone would actually just carry a sword(not to mention about getting in proper medieval outfit of a fighter) on those circular stairs and then when resting and catching breath thought about advantages of sword fights on STEEP staircases, regardless of their direction...