Don’t use too many fasteners. One large bolt is better than many screws or nails. You get the same strength but with fewer puncture wounds to the tree. Whenever possible, perch your tree house on top of fasteners rather than pinning beams to the tree. This gives the tree room to move and grow. Even for smaller, lighter tree houses where the load is spread over three or four attachment points, consider using 1-in.- or 1-1/4-in.-diameter lag bolts.
Built for Sunset's book "Kids Places to Play," this is an example of how a treehouse can be built with all off the shelf materials (except for the salvaged windows, cedar pickets and 2 fabricated nave brackets). The route to the crows nest is thru a trap door. There is ample room for 2 kids in sleeping bags. Via treehouseworkshop.com