Given how this tree is shaped I couldn't really wrap around the trunk; though I didn't like the idea of having a hole in the roof for the trunk anyway. So I decided to build the fort off to one side and have a deck off the other.
I bought two GL's, as they're called, to hold up the two 4x6 beams you can see horizontal in the picture on the left. I then used two 4x4 posts to create diagonal braces to hold up the fort side of structure. It was a challenge to connect the diagonal beams but I thing I arrived a reasonable solution with some advice from a friend.
The next step is to finish laying out the jousts and putting the decking and floor down so that I can start having the boys help in the construction. I'll post more progress as I find time.