Just a note on the steps. So far, the steps have been the trickiest part of the treehouse building process. My first version was badly slanted and not especially secure (a fact I found up when I began hauling equipement up them). I tried re-do some of the stairs. Not that it easy to do, as the ladder weighs a ton and wrestling it into position is dicey with that weight.
After doing some quick web research, I decided the easiest approach was to build them in place. So, I tore down the old one and installed two slanted 2x8" lengths of PT wood (beveled on the feet). Once in place, it was a simple matter to cut the trends to fit and place them singly. (although I noticed when I was done that the bottom tread is inch and a 1/4" wider than the top tread, so much for parallel sides!)