When I was over to the home of my brother-in-law recently I saw that their upstairs-neighbor had built a cat ramp from the ground to their terrace (1st floor, b-i-l is on entry floor)... would something like that work for you?

Then you could put a cat flap in the terrace door (or a window besides bedroom if you can put the ramp to that) for the cat to come in and out.