She now sells enclosures in dozens of shapes and sizes, some with fanciful names like the Penthouse (with three tiers of hammocks) and the Ferris Wheel (it looks like one). Kittywalk also sells strollers for cats and dogs. And as a support-the-troops gesture, the Kings have sent cat enclosures to American military bases in Iraq and Afghanistan.

“The thing that gives me the most gratification is that cats like my products,” Mrs. King said.

Even indoors, a cat enclosure can be a boon, said Carole C. Wilbourn, a cat therapist in Manhattan who recommends them to clients struggling with what she calls “inter-cat hostility.”

“I have cases where someone is introducing another cat, and they have a studio apartment,” Ms. Wilbourn said. “It’s kind of hard for them to put up a barrier” for the cats to get used to each other without fighting, she said. An enclosure keeps the cats separate — but within eye range — until they get along.

VETERINARIANS disagree over whether it is depressing for cats to spend their lives indoors. Some, like Drew Weigner, a cat specialist in Atlanta, believe that outdoor space offers cats emotional benefits. While it is safer for them to stay inside, “in an enclosed yard, they’re going to get more exercise,” Dr. Weigner said. “Plus, there’s the intellectual stimulation, quote-unquote.”

Dr. Weigner advises suburban cat owners to keep outdoor enclosures off the ground, to guard against fleas and other parasites. He pointed to the American Association of Feline Practitioners’ Web page on environmental enrichment, which recommends several types of fencing for suburban cats.

Arnold Plotnick, a veterinarian who owns the Manhattan Cat Specialists practice on the Upper West Side, agrees that stimulation is good, but does not think that cats need to go outside. “Cats are really fine being indoors their entire lives,” he said. “That’s why they’re the perfect New York City pet.” But he warns about high-rise syndrome, in which cats leap or fall out of unsecured windows.

For cat owners in the city, there are a few off-the-shelf options. On the low end of the market are collapsible, stand-alone enclosures that can be used on a terrace or deck (or indoors) and cost as little as $40. For people with small yards, there are room-size enclosures — typically with a few shelves where cats can sleep or look around — that can be bought or built for $125 to $500.