The tree fort is the tree house where Calvin and Hobbes are often seen. This is the location of all meetings of G.R.O.S.S., except for the first meeting which was held in the garage, and the last meeting, which was held under the Box of Secrecy. Calvin seems to think he has a advantage over Susie up in the fort, as shown in several strips where he is shown throwing water balloons at Susie from the fort. He is also commonly seen plotting against Susie in the fort, such as in one strip, in which he plans to steal Susie's doll. This fort is also the site of many fights between Calvin and Hobbes, which often result in cartoonish clouds of dust appearing out of nowhere as they fight. Stupendous Man has used this tree fort as his secret fortress when hiding from Babysitter Girl. Over the year, this fort has been the site of many stories, although its primary use is always to serve as a place where Calvin has absolute control, even if Hobbes takes it over on a regular basis.
Bill Watterson has said that this treehouse came along with his inspiration for G.R.O.S.S. That is that it was based on real life experiences, of him and his neighbor having a club to annoy the neighborhood girls. They would plan for tons of things that would never happen, and perhaps the biggest inspiration for the tree fort itself, they would hide a suitcase full of acorns in a tree and wait for a crisis that never happened so that they could use it.
Calvin has to say a password to Hobbes if he wants to get in. Since Hobbes made the password because he doesn't need the password, it is an ode which praises tigers.