In my experience, city horse keeping means horses kept in stables with no turn out. Think police horses, think cavalry army horses, think delivery dray horses. However, such animals are working for several hours a day and they are also fed appropriately. They will have vacations for short periods, turned out on grass, largely to rest their legs. Then I have experience of horses kept at grass who never enter a stable but are equally well adapted, if somewhat shaggy and muddy.
One really important variable in “the best” way to keep a horse is how much work it is doing. Turn out allows constant movement, which is probably the most “natural” thing but a horse putting in four or five hours of work each day doesn’t need that exercise.
A horse banged up in a stall for 23 hours a day in a dark barn and fed like a racehorse is going to require a lot of management. Think show barn.
Historical note: 19th century leisure horses kept in London were exercised for two hours daily, by their grooms, so their owners could have a pleasant hour or so hacking in the Park later in the day.