I made my cats a play house many years ago. It has two rooms with a window between, One room enclosed on three sides, the other open, and a cave. The 2x4 is for wood scratchers, but also allows room for expansion either out, or up, should I ever live in a place with more space. (It's getting a bit shabby now, I made it about 10 years ago) I didn't use plans, but there are plenty available
They also have a stair step perch
I also made a window tower for my cats, with shelves and a hammock, using plastic tier shelving. It is anchored to the wall for stability