I did do a bit of research on the Net before starting this project.
Pine trees are all I have on the propertyso it was a
pretty easy choice when it came to tree type.
Now after one year and a lot of wind I can say, in this
case anyway, pine trees are strong enough to support a heavy load.
At the base these two trees are around 24 inches wide.

The one thing I took away from researching was to over-build.
We had one strom that was so strong I could not look out back until it was over.
I really thought I was going to have a mess in my barkyard.
I consider that one its grand test.  I'm sure now
that the trees may up-root, but the treehouse itself is not falling down!
 

Get a good set of power tools.  A circular saw is a must have.
A good drill is also another have to have.  I was able to reach
the treehouse with a 100' extension cord.
I prefer regular power tools over cordless.  Just plug in and go.
A rope and bucket is great for everything.  Nails, tools and drinks.

I was able to use a ATV with a 50' cable to do the lifting.
In this picture you can almost see the cable running up
along the ladder.  The supports holding the brace before its attached were
used just to make it a little easy to get it
into the correct position.  It's still being supported above by the cable.
By the last brace I had this down
to a science.  Each brace was put together on the ground.

The underside of the deck. The extra supports, besides the main
braces,  were added to get rid of a small amount
of bounce.  It also adds a not so planned-out look, which makes
people look twice and ask if it's safe!
Running over the tops of the main braces are 4x4's and 2x4's.
The 4x4's are lag screwed into the trees.  2x4's for decking
complete the deck.
 

Getting ready for a big lift.
 
 

Looking up from below.  That's the trap door closed.