For all the cat lovers...
Last year I built my daughter an indoor Cat Condo - this year she wanted an outdoor enclosure connected to the house so the cats could come and go as they wanted. Thus began the "Catio" - it is a basic structure with wood (actually siding) at the bottom and wire fencing at the top. The roof and the tunnel between the house and the "Catio" are plastic roof sheeting.
The tree was copied from a photo online showing an Ikea wood tree - I just added some texture, a knothole, and a cutout at the bottom for a little mouse to hang.
A ramp leads down from the entryway to some shelves and steps lead up to the top on the other side, near the tree. My daughter wanted grass for them to chew on, so I bought a mixing tub from Lowes and enclosed it to allow them to sit along side the grass (grass not planted yet).
I still plan to add an enclosed bench (with cat entries) for the cat boxes and to provide seating when petting the cats. I'll make the front lift-up, so they can pull the cat boxes out for cleanup.
Because I couldn't work on this full time, it took me a little over a month to finish.