Come on guys, you don’t need a fencer to tell you about this, you need someone with a background in history, specifically historical castles and castle defenses.
Stepping forward, since medieval architecture is a hobby of mine.
Its not a myth, its a fact.
There are almost no ‘left handed ascending stairs’ (stairs that spiral upward counter clockwise) in any castle or construction that was considered for defensive purposes. Reason, its much harder in the era of edged weapons to attack up a stair turning clockwise than counter clockwise.
Why? Because most of us are right handed. We would hold our edged weapon (axe, sword, whatever) in our right hand.
Now, stand in the narrow spiral stair with your blade in your right hand looking up. See how the inside of the spiral makes you step out and expose your body to the defender above you because your hand is too close to the inner spiral. It also prevents you from getting any effective cutting angle on the defender above you, swords don’t bend around curves.
Now, do the same thing at the top of the stair looking down. Now your weapon ins along the outside wall. See how much more angle of effective area of attack (or defense) you can get by just exposing your arm and weapon and how the curve of the inner side protects your body?
With very few swords truly left handed, it means that the attacker is at as much disadvantage as possible. He may use his left hand, but not be as good. Or he may have to use his right hand and be at a real disadvantage.
But left handed defenders were just as hard to find, so its likely that the left ascending spiral was a stair not meant to be defended, or perhaps built at a later time when active military defense was not as much a concern.
The biggest builders of castle construction in the British Isles were the Normans. Its how they pacified an area. I toured many across Ireland. Almost all were living residences as well as administrative centers for the local Norman lord. Even in the residential areas, this stair construction holds true. In addition, even in the residential areas the stairs had random riser heights - that is the stairs were uneven. Also, they edges of the stone on the stair treads was left intentionally sharp. You had to pay attention and not screw up or you easily fell. Something else that causes an attacker to have to slow down.
There you go!
William