I think you mean "cobble" not hobble.

I'm with steliart and Magickaldan.
Please please double up your vertical posts.
They're supporting a lot of weight, especially with kids or adults in the treehouse.
Just bolt equal length posts to the ones you have already.

you could spread a small tarp on the top for a roof

 I will agree with Magickaldan your posts are weak and will get weaker after a winter. Not safe at all, especially for kids. It is clear even from the pictures that you need to at least double them up.

Other than that a very well documented instructable

 A classic!  This is how we did it when we were kids, only without the concrete, planning and adult supervision.  I miss those days...

Should of used 4x4 posts, and I would for the flooring have turned the 2x4's so the widest part is vertical the way you have it now it's only a matter of time before they splinter and break. 2x6's would have been ideal.  But good job documenting it.

It's great, but I'm curious about the fence - galvanised-steel? What spot of land is this?

L