My parents’ house backs onto a golf course, with a good 25m of mature woodland at the bottom of the garden. I always wanted a treehouse when I lived there, so over the weekend I realised my dream, but for my kids instead 
Design:
The whole upper platform rests on the bottom of the diagonal braces The feet of those simply rest against the tree, while being suspended from some loading straps which hang from a fork in the tree. No nails or screws in the tree. The lower platform is built like a normal deck. The ladder/steps brace the upper platform and prevent it from swinging around.
Build:
The client inspects the site and approves construction
Me and my old man. The deck was built around the tree on the ground
We strapped a long 2x4 to two trees to make a lifting beam. Two main lifting points, with the sideways motion restrained by a third rope tied back to a another tree.
The clients are happy with the project so far.
As am I
It’s more or less flat up there, and very solid. No sway or wobble.
The railings will have to wait for another weekend…