Most cat tunnels are designed to be used indoors, although you can certainly use a cat tunnel in your garden if you want. Just be sure to secure it to the ground and keep an eye out for rain, so it doesn’t get ruined. There are even special zip up tunnels that are meant to be used outdoors. However, if you want your kitty to go outside more often and for longer stretches of time, you might want to look into building a cat run. These are designed specifically to allow your cat to lounge on the lawn and get some fresh air, and typically tend to be well-anchored to the ground.