love this attachment from the house to the cat pen.
DIY Wall Cat Run - PetDIYs.com
outdoor tunnel system for your indoor cat: Top 20 Brilliant DIY Backyard Projects and Tips for Your Pets
Cat Enclosures Seattle - Catio Spaces: window box
I don't need window access, since my house has a little doggy door, but I do like all the levels in this one.
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A Catio is the Coolest Thing You Never Knew Your Cat Needed
Kitten Cubes Cat Condo von TheHeftyCatCondo auf Etsy