While we were there, we came across this house - the woman there had fenced up her entire front door so the cat could not go out. This may not be the prettiest way of doing it but it's fast and quick to attach - and most importantly, keeps the cats in.
She was also a fount of information, letting us know where all the cat owners in the block of flats lived. We spoke with most of them (those who were at home) and most had their cats sterilised and seemed to be kept indoors. One woman though had a cat that was peeking out the grilles and when we asked her, she said it wasn't sterilised yet but that it never went out. When we walked a few steps down the corridor, the cat-that-never-goes-out was seen in the corridor. The woman quickly came out and scooped him in and closed the door.