It is hard to give a very definitive answer because cats differ so much. I am keeping two cats in a second-floor flat here in Limassol. Both were rescued from the street as small kittens and are most probably of feral parentage. They have now both just turned three years.
One of them is a perfect indoor cat. He has no desire whatsoever to leave the safety of his little world, and will struggle to get down if you even try to carry him over the threshold. He runs and hides if any strangers enter the house.
The other character - pictured below in one of his favourite sleeping positions - is very inquisitive and a great explorer. You only have to open the front door and he will dash out and make his break for freedom. Having brought him up as an orphan from the tender age of two weeks from a bottle, I had hoped to keep him in a sanitised, indoor world. However, it was so obvious that he was yearning for the great outdoors that I started to take him out for walks. This is something that he began to relish, even beginning to make a huge fuss when his regular walk time was approaching. Luckily there was a large area of waste ground directly opposite our block, and I used to take him out there and walk along with him, directing him away from fenced-off areas so that I could keep a firm eye on him. I began to notice that he had a lot of very sound survivial instincts, which no doubt come from the local feral behavioural gene pool, the chief of these being a fear of the sound of cars and lorries. He will always run in the opposite direction when he hears this dreaded sound. Sometimes he would interact with local feral cats in the streets. From what I have observed, the two cats will face one another off for a while, then one will run off. Sometimes my boy was the one to run, at other times he was the one who gave chase. Again, his familiarity with this kind of behaviour may have a lot to do with sharing genes with the local ferals. Nowadays, I just let him go off for his walk in the confidence that he will return in about an hour. Fortunately, this a less densely built up suburban area so there is waste ground for him to explore - although it is gradually being built on and the waste ground directly in fron of us has partly disappeared. I just keep a look out from the balcony and sooner or later he is standing below, hollering to be let back in. I would not feel comfortable advising you to do the same thing with cats that are not adapted to local conditions, though. I have become resigned to the fact that this fellow will probably not die a natural death, but on the other hand I feel that forcing him to stay within foor walls all the time would be cruel. The fact is that he has already outlived the average life expectancy of a feral cat in Cyprus, so he has not done too badly in being found by me.
Talking of balconies, this is the major bug bear when it comes to keeping cats in flats. Do not assume that they will not fall off, because they will. When both of mine were small kittens I once took them round the flat and held them off every balcony and out of every window and let them squeal with fear for a while in the hope that this would teach them that there were high drops from these places and they should not jump here. Until a month ago I was living in a fool's paradise that this technique had worked until the cat pictured below fell/jumped off one of the balconies and fell about 10 metres onto solid concrete. Unbelievably, he escaped with no more permanent damage than some cuts around the lips. It is true what they say about 9 lives! I have now had to get some expensive building work done to cat proof the balconies. This is something that requires serious thought.
Another point that you should consider in deciding whether to let cats out or not is lanate poisoning. The use of lanate to keep the cat and dog population down is widespread in Cyprus. There are many stories of people having their pets poisoned in this way, and by all accounts it is not pleasant to watch your pet die in this way. This is something you should be aware of.
All in all, I would say that it would be difficult for cats that are used to going out to adapt to indoor life in a flat. I really wonder if cats that do not have feral genes in them could safely negotiate the perils of the great outdoors in Cyprus. If they are used to going out, they must have honed their survivial instincts. It is hard to say.
I can't really give any definitive answers, but hope the above has at least given some food for thought.