When Peter Cohen bought his house, it came with a couple of roaming outdoor cats. After tragedy befell one, and nearly the other, he began rescuing cats and bringing them inside his Goleta, California, home (near Santa Barbara). Before he knew it, he had well over a dozen indoor cats.Video: See the cats in actionTo keep them occupied, Cohen, a home builder, began designing and constructing elaborate catwalks, tunnels, platforms and perches throughout his house. So far he’s spent nearly $40,000, including for specially designed closets with exhaust fans to keep litter-box odors at bay. Meanwhile, five Roomba robot vacuum cleaners prowl the floors for cat hair.In the home office, cats can climb up a rope-wrapped pole and either hop to adjacent platforms or descend a spiral walkway. “I thought maybe the cats would just look at it, but they use it all,” Cohen says. “It’s like a freeway sometimes with them all over the place.”