This is the second accessory I purchased and I guess I should have read more of the reviews, because if I would have known the dome comes with sidewalls I would not have purchased the sidewalls first then later the dome. I would have paid more for the dome and had the option to use the dome or just the sidewall. So if you are unsure just save your money and buy the dome. You can start out with just the sidewalls if you think your cat might not like it, but either way you will have options. I have two cats a calico and a Maine coon so it is pretty large as my Maine coon is about 18.5 pounds. The price is only 15 more dollars for the dome which comes with both parts compared to just the sidewalls.