A Tree Runs Through It--Tree house Tree House Designs - Ideas for Treehouse for Kids - Popular Mechanics
Floating bracket support Allow for flexible supports, especially if you use more than one tree, so that trees can move in the wind. Special floating brackets allow the tree to sway.
Racheting straps around tree truck, shaped plywood stairs, and padding on tree side for an adaptable spiral stair case into the trees upper reaches. CanopyStair by Thor ter Kulve and Rob McIntyre