The tree house is coming along. We have the windows installed and it is time to start on the siding and the roof. The roof really holds it all together and strengthens the walls.
Here is the tree house from below showing the walls and the newly installed windows.
A view of the windows from the inside. I got these windows from a Habitat for Humanity re-sale store for about $5 each.
Working on the tree house at night. I used steel clips to attach the 2"x4"x12' boards on their sides to the top plates of the walls. 
The roof frame is 2"x4"x12' boards to support the 1/2" treated plywood sheets. I have a 3' overhang over the deck on front and a 1' overhang on the back.
Most of the siding is done with the exception of some corner trim and soffit work, but it is starting to look like a functional tree house. The kids are starting to use it for play.
Drip edge is installed prior to tar paper and shingles on the roof.
You will need help to get the heavy tar paper and shingles onto the roof. We fashioned a pulley system to a tree limb above to lift the materials to the roof.
Tar paper is laid down and nailed with roofing nails in an overlapping fashion starting at the low end of the roof working your way to the top, so that water can't get under the layer below the top layer.
The same goes for shingles. Starting at the low end you overlap up to the top, so that you cover up the nails of the shingle layer below. The shingles and the tar paper have lines on them to keep everything straight and to make sure you keep the appropriate amount of overlap.
Here is where we are today. I still need to finish corner trim, do some soffit work, insulate (if we decide to), install the wood stove and finish off the inside, but for now we have a functional tree house. Hopefully we will add solar panels soon, but for now we have an extension cord supplying light at night. I hope you have as much fun as I have had.