There was an article in the New Scientist (YMMV) about this subject about 30 or 40 years back. It described research that found (for what was thought by the researchers to be the relevant period in history) just slightly more that 10 percent (not 30% as this article asserts) of staircases were left-handed, that fraction being approximately equal to the proportion of persons or presumed castle defenders who would have been left handed. It drew the inference, compatible with the idea that castles, being large, might require a large number of defenders and would contain a sufficient number of staircases, that defenses would have been optimized by allocating defenders to staircases according to handedness when the aforementioned fractions were equal.