My sister used to live in a 1st floor flat. The door was at the front, almost onto the road, there was a bit of grass at the back (and then a field, maybe, or some allotments, I can't recall) Her cat would go out of her flat and yell at the door or the downstairs flat. The man who lived there would get up and open the door. She would walk through his flat and he would open the window for her to get onto the back (she could have walked round, but I assume she couldn't be bothered!).
When she wanted to come back in, she would sit on his windowsill. He would open the window again and she would walk through his flat, he would open his front door and she would go through and back up into her own home (although sometimes she would sidestep into his bedroom instead and go for a nap!)
This cat was someone you did NOT mess with! All claws and teeth!
I'll never forget when my sister told me how she had fetched the cat in one night...
"I saw her across the road so I went to pick her up. I grabbed her and she started hissing and spitting, so of course I didn't think anything of it. Then I looked across the road and there was MY cat.."
My own cat is very daft (or maybe very clever!). She keeps forgetting how to use the catflap and will sit in front of it, waiting for you to flip it for her.