I have lived in a city flat and had cats that were predominantly indoor cats. I took them out for walks on a harness and lead.
If you get them as kittens and keep them indoors, they get used to it,
I wouldn't say it is cruel to keep a cat in this way, but it is unnatural,
However, letting your cat come and go when it pleases does entail certain risks.
Not all your neighours will appreciate the cat going into their garden, nor will they believe you, when you say that most cats, even if they have a litter tray at home, will see no need to run home and use it, rather than dig a little hole in whichever garden they happen to be in when bodily needs have to be met.
Traffic is a risk - our last cat - a former stray who had looked after himself for a couple of years before moving in with us, was killed by a car. To date he is the only one of the 13 cats that have accompanied me on my journey through life that has been killed in traffic, but it is a risk.
A cat that goes out can just disappear - this happened to one of my sister's cats. We never found out how or if he had died.
Other risks are the cat fighting with others, being bitten by a rat and the bite turning septic, or being chased, even killed by a dog.
So if you decide to let the cat lead a free and natural life, you do need to expect vet's bills for treating accidents, vaccinations, as there is a greater risk of the cat contracting an infectious, perhaps lethal illness, and for having the cat chipped or ear tattooed, so you can prove ownership if it gets lost and is found (alive or dead). And you may be facing finally never knowing what happened to a beloved pet.
So although I personally agree the cat probably prefers the more natural life, it is risky.