If you don't have the space or the right setting for a regular catio in your yard, it's still possible to give your cats a  way to enjoy the sights, sounds, and smells of the outdoors with a small enclosure mounted on the outside of a window.

This window catio in a condo with a small back yard uses nylon netting that allows minimal obstruction of the view out the window but still keeps kitties safely contained.  If you look closely at the lower left corner of the open window, you'll see a cat door that mounts in the window screen to allow access to the catio but keeps insects from getting in the house, and the window can still be opened and closed normally.

This enclosure sits on a shelf outside a small bathroom window.  The window screen was replaced with a plexiglass panel that allows a cat door to be mounted in the window.  Strong wire mesh was used on this catio because raccoons from a nearby creek often climb up on the shelf outside the window so security from them was a top priority.