That is an interesting point, my experience is with far later fortifications. Firing from the narrow side means you only have to move slightly be be protected from all angles at the opening, and only have to move a little to change where you're aiming at. Narrow opening at the outside means you have to take a step to either side to see the entire field of view.

I think where this is most noticeable is in modern fortifications, the large concrete bunkers of the Great Wars, with their stepped framed openings. Basically the gun ports would be formed by using a stack of wooden forms, each one smaller than the last by a bit, making the sides right angled steps moving inward. Anything shot at it would catch, and any deflection would likely knock it into the far side to be caught, and not into the defender.

Sadly the few books I have on castles from earlier years, (which would post date Conway Castle.) and showed the narrow side inward, are all packed away in god knows where now. I wonder just when this switch happened, and why.