Is it cruel to keep a cat in one room?
If you asked a hundred, knowledgeable animal and cat loving people whether it is cruel to keep a cat in one standard room I would expect at least 90% of them to say that it is cruel. It does however depend upon the environment created and the size of the room. However, this is not a black-and-white situation and you can’t go off at the deep end and decry a person who keeps their cat in a room. It also depends on how long the cat is kept in a single room. Indeed, when you introduce a new cat to existing cats in a household it is advised that you keep the cat in a room for several days to allow a gentle introduction.

Picture (unmodified) in public domain.
But if a person told me that they wanted to adopt a cat to live in their apartment and that they wanted to keep their cat in one of the rooms I would advise them to not adopt a cat. Why would it be cruel to keep a cat in a room permanently?
Territory – home range
The first reason is that if we are to respect cats we have to allow them to express their natural desires and it is unlikely that they can do that in a single room in a flat unless the room is 50 feet long and 30 feet wide with a catio attached to it. Domestic cats need, ideally, around an acre of territory – home range.
Okay, they are highly adaptable and can live in smaller areas and they can do so reasonably contentedly but a single room is simply too small for a cat to feel comfortable in. Yes, a domestic cat can live in a room. The cat will survive but you won’t know how tormented she’d be. It may cause mental health issues. It is likely to make the cat very timid and nervous about any form of stimulation unless a great effort is undertaken by the owner to make the environment adequate. Arguably, though, it is impossible to make a single standard sized room environmentally adequate for a domestic cat.
A single room in a home might be less satisfactory than a cage in a zoo and we know that zoos are not good places for cats as they are known to be stressed. At least the cage is open to the outside environment where the animal can smell, hear and see what is happening around him or her. A room may have a window which may allow the cat some stimulation from outside activity but it is entirely inadequate to depend upon this.
The bottom line is that a single room does not (1) provide enough space for a domestic cat to satisfy innate desires and (2) will lack sufficient stimulation by its nature. You can test the question by taking it to an extreme. If you place a domestic cat in a 4×3 foot box for the remainder of his life you would be prosecuted for animal cruelty because everybody would deem it to be very cruel. Therefore it’s a question of size and a single room is, I would argue, below the threshold for an adequate size of territory and nature of habitat for the typical domestic cat.