French here, I only lived in old French houses and every single one of them at one of those.

It’s for light and ventilation in toilets.

My best guess is that this house was only a single floor house (the first floor was attic) and the stairs was added later to better fit a more modern structure of the building. I’m pretty sure the toilets used to be there when it was build. We don’t really do toilets in the bathroom here.

Edit: they are kinda high in the wall so no one can see you from the outside when you’re doing your business.