Peter Cohen was inspired by Bob Walker and Frances Mooney's book The Cats' House. After a remodel, "I thought, ‘I’m a builder. I could do that.’"
Plenty of pictures at these links:
Houzz TV: Watch These Rescued Cats Make a House Their Playland
Spirals, catwalks, tunnels and platforms create a superhighway inside this home in Southern California
Incredible Home Catwalks Make for Purr-fectly Happy Felines
Walkways and tunnels custom built to the tune of about $35,000 keep a homeowner’s 18 cats on cloud nine (Houzz)
The playland consists of dozens of platforms, tunnels, nooks, spiraling ramps, and holes that look like a shark's gaping mouth on which the cats can scratch their backs. It even has closets with filtration systems and exhaust fans to hold the many litter boxes as well as constantly cleaning Roomba vacuums.