Designing - Beam, Joist, Floor and Stud Plans

I have been planning, drawing, erasing, redrawing and planning some more all Winter.  I am trying to figure out the optimal design of the tree house with the lumber that I already have so I do not have to buy too much.  I think I have a pretty good idea of how this is going to look.

The first plan is the Beam Plan, which is technically the foundation of the whole structure since everything sits on top of it.  I am going to use two 2x12 boards parallel to each other.  I am going to add cross members as room permits to act as stiffeners so this becomes more of a box beam (per the suggestion of the eternal engineer - my Dad).

Here is the plan as I currently have it drawn.