Cats are generally very good at balancing on ledges, fenceposts, windowsills, etc., but of course they do miscalculate now and then. Better to be safe.

I've had some episodes of having to put cats in one room so I could air out another without tempting them to explore outdoors. Then there was the episode when my more obstreperous kitty *knocked out* the bathroom window in his quest to get outside when he was annoyed, fortunately a window only a few feet above the ground. Sturdier window had to be installed, but that's fixed it.