The treehouse first made an appearance in "Do Not Fort Sake Me". Where no cats amd Millie are allowed.
The treehouse is 50 feet high, has four floors, and is equipped with a bowling alley and a high tech security system, which Kat used against Coop and Dennis. The builders Are Coop & Dennis. It is made of a fence that Coop & Dennis take down. Burt tries to stop them but he fails to do so. There is pirate flag in there and a TV.