I'm about to move back to Paris with my cat after having spent years in North America. My cat was found in Pittsburgh and has lived with me there, in California and in Montreal. In all of those places, there were two things in common: buildings were low and there were always window screens. When he was young, he absolutely wanted to go out, but I had a strong feeling things would end very badly for him if he did, like getting run over by a car, get lost or a combination of similar things.
But in Paris, there are no window screens. And we'll be at what Europeans call the fourth floor and what Americans call the fifth floor. (We count differently.) The living room has a spectacular view over one of the main plazas in Paris, which I thought would be great for my cat to have something to watch over. It also has this sort of rail with a wide but somewhat slippery top. My big fear is that he tries to jump onto the rail, fails and falls over down the five levels. For a cat, he is sort of clumsy. I am planning to put a cat tree in front of a living room window so that he can look at the view more easily, but in front of a closed window only. The kitchen window is at the same level, does not have a nice view, but I'd still be afraid he'd fall. He's about 7½ years old, and still pretty curious.
It will be fairly hot when I arrive and I will need to leave some windows open. (No A/C in Paris.) I know it's just a detail and my cat will probably know better than fall five floors out a window, but when I was a child, I had a friend whose cat fell from the fourth floor while pregnant and lived, but lost her kittens. They had a faulty balcony that no one was allowed to go on. The balcony broke from under her, I think. She did not fall actually fall from the window.
When he was younger, if I told my cat to come back in a stern voice if he was about to do something dangerous, he'd just run away faster. But now he trusts that I have his best interest in mind and actually backs off from what he's doing and runs back towards wherever "home" is. This took years. But these windows still scare me in advance. And I know that for all cats "no" usually means "wait until I'm not watching over you."
I have a friend whose cat started walking on a ledge on a Paris building and visited other buildings. He actually did okay with that until he was run over by a car in the countryside. I don't think my building in Paris has ledges.
So any advice of what I should do would do when we arrive would be welcome, even if it's "don't worry: he'll know not to fall from five floors above ground."