Once your pet cats are litter trained, you will realize how dependent they are on you as their providers. Cats unlike dogs cannot be walked. Some people do but it comes with its share of pros and cons.
That said, every activity of the cat revolves inside the four walls of your home. If you live in an apartment and your cats do not go outside to relive themselves, you will have to arrange for a suitable space, props and material to mimic the outdoors.