This is the cardboard TARDIS cat playhouse made and sold by Etsy shop Cacaopets. They're easy to assemble (allegedly -- although I'm sure I could screw it up like my last IKEA couch, which now has an armrest between the cushions that makes napping impossible), just about 20-inches square, and cost $55. Knowing my cat, he'd climb in there, forget how he got in, then panic and break one of the windows to get out. He's not the smartest cat in the house. Just kidding, he's both the smartest and dumbest cat in the house, because he's the only cat in the house. Just like how my brother always says I'm the best and worst brother, because I'm his only brother. "I never said you were the best." Those words hurt, Frank.
Keep going for a handful more shots.
Thanks to Dunc, who agrees no matter what, your cat is still going to spend more time trying to squeeze into the cardboard box the TARDIS was shipped in.