When the floor is full, the walls become the cat's territory. Shelves, perches and climbing routes give a cat height and a lap of the room without taking a square foot from you.
Cat wall shelves ideas covers spacing, studs and weight ratings, cat climbing wall ideas links single shelves into a route, and cat window perch ideas puts the best seat in the house where the light is. If drilling is off the table, floor-to-ceiling cat tree ideas uses a tension pole instead of the wall.