Start with boxes your cat can actually move through
Collect sturdy boxes big enough for your cat to enter, turn around and sit upright in — small delivery boxes that force a squeeze will be sniffed once and abandoned. Three to five large boxes make a good first fort: one entrance chamber, one middle tunnel box and one dark resting chamber at the end. Cut door holes wide and low, roughly the width of your cat's body plus room for whiskers, and smooth every cut edge with your fingers so no sharp corrugated ridge scrapes her sides as she passes.
Remove all tape, staples, labels and packing material before she goes in, and skip any box that held cleaning products or smells strongly of chemicals. Join boxes by overlapping their flaps and cutting matching holes where they meet, rather than taping seams she could chew loose. Place the whole fort on a non-slip floor away from heaters, with its main entrance facing the room so she chooses to enter rather than being placed inside.
Design routes, not just rooms
A single box is a bed; a maze needs decisions. Cut two doors in the middle chamber so your cat picks left or right, make one route a short dead end ending in a scrunched paper ball, and make the longer route reach the resting chamber. Add a lookout hole at head height in one wall and a low peek slot in another, so she can monitor the room from cover — watching unseen is half the pleasure of a fort. Line one chamber with a worn T-shirt that smells of you and leave another bare cardboard, then notice which she naps in and keep that texture in the final chamber when you rebuild.
Keep the fort single-storey, stable enough that a landing pounce does not collapse a wall, and away from shelves a bold climber could use it to reach.
The daily change is what buys you days of interest
Cats explore novelty, not square footage, so the fort stays interesting only if it keeps changing in small ways. Each day, alter exactly one thing: block yesterday's main door with a flap and open a new one on another wall, rotate a chamber ninety degrees, move the treat chamber to the far end, or add a hanging strip of paper just inside an entrance for her to bat through. Small changes force new route-finding without destroying the familiar layout she has mapped and trusts.
Seed each rebuild with two or three treats or a favourite small toy placed deep inside, never at the entrance, so the reward requires the full journey. On some days hide nothing edible and instead put a fresh cardboard scratcher panel in one chamber — variety in what a room contains teaches her to check every chamber every time.
Retire and refresh before it becomes furniture
After a week or so the cardboard absorbs odours, claw damage softens the walls and your cat's interest will dip even with daily tweaks. That is the signal to recycle the tired boxes and build version two with a different floor plan, keeping only the smelly T-shirt liner as the familiar bridge. Watch for chewing and eating cardboard rather than shredding it — remove any box she persistently ingests pieces from and mention ongoing cardboard eating to your veterinarian, since it can occasionally point to a broader chewing drive worth checking.
With KinsLe Care Teams, the daily maze tweak, treat seeding and box-swap jobs can be assigned to different people and ticked off with a timestamp, so the fort your cat loved on day one is still earning exploration on day six.