My treehouse was a 4x4 sheet of plywood with 2 2' 2x4's cut for bracing stuck/nailed in the crotch of a tree.<p>no roof, no walls, no rails. random rope to help climb the tree to get on the platform... I guess it wasn't so much a house as a place to play up high.<p>I knew some guys with 2 tree houses in different trees, way way to high, connected via ropes. The guys were almost done with highschool, and didn't use them. If we'd been about the same size/age, would have been a blast. As it was, i got to climb up and across a dozen times or so.