If you use a handweapon (sword, axe, knife, hammer, etc.) you need space to perfom a swing with any meaningful amount of energy. Medival stairways were built t

If you use a handweapon (sword, axe, knife, hammer, etc.) you need space to perfom a swing with any meaningful amount of energy. Medival stairways were built this way because people going upstairs won't have the space to swing if they are righthanders. Meanwhile people going downstairs have a good amount of space to do precisely that. The assumption of course is that you as the defender are the one fleeing upstairs, while the attacker is trying to get to you. Source: grew up around medival european castles and have been on a few tours. When you are inside such a stairway this becomes very obvious.