when I visited cahir castle a couple of years ago, the tour guide told us that the stairs in the castle are designed so that the attacker climbs up the stairs in a clockwise direction.
Majority of people are right handed and climbing in this direction puts his sword hand to the wall side of the stairs.
The defender, also right handed, is free to swing his sword
The steps in the stairs are also deliberately uneven in height to hamper an attacker.
Photo of castle from google