I had two house cats when we lived in a flat. All you have to do is not let them out! but make sure they can see out of the window as it's interesting to watch outside. Very clean litter tray kept in a different part of the house to the food area (we kept out little trays in the bathroom). Cat grass or catnip in dishes give a change of snacks - ctas would eat grass out of doors when they need to. Mind your house plants - some are poisonous to cats. If cats don't do out, they don't mind. My friend made a wire screen so she could open her windows wide without worrying that her casts would jump out.