The Tree Fort is located right under Jimmy Elliot's backyard. The origin of it is unknown, but when Liam first met Willy Barna, Willy gave him a tour of the neighborhood that concluded with the tree fort. There were several pieces of wood on the tree, a mattress mounted on a sturdy branch, and a bucket which Willy said that they "used for pooping". They really didn't, but Willy's little brother Joey decided to prove his point by actually pooping in the bucket.
Sometime in 1998, Jimmy and Marcus made a verbal contract giving the tree fort to Liam. They later denied this ever happened.