Happiness cannot be reduced to a universal indoor-versus-outdoor verdict. Outdoor access may offer exploration and choice, but it also brings hazards; indoor life can be secure yet needs deliberate provision for activity and stimulation. The individual cat's history matters. A cat accustomed to a large outdoor territory may respond differently to confinement from one already comfortable indoors. Look at patterns of eating, resting, play, grooming and interaction rather than judging from a single behaviour. Persistent pacing, withdrawal or other changes deserve investigation rather than an immediate assumption about the cause.
Health problems can also alter behaviour. Aim for an environment with safe choices and dependable care, and ask for professional advice when you are unsure. The useful question is how well your particular cat is coping with its current life and what practical changes could improve it. Choose one change to evaluate, such as a quieter resting area or scheduled play, and record the response rather than changing the entire routine at once.
For product comparisons alongside this advice, see Paw-Villian cardboard scratcher and Mouse Chase play set. The details below explain their sizes and uses.
Further reading: rspca.org.uk; cats.org.uk; pdsa.org.uk; pdsa.org.uk.
Paw-Villian cardboard scratcher
Mouse Chase play set
Koda Pod