Fairly simple tree house wrapped around a single tree.
Some great pictures detailing how a tree house can be integrated in and around a tree.
[via] http://thewildhare.blogspot.com
Plans In FULL Color With A Picture Of The Final Result
A Tree House
A tree house, a free house,
A secret you and me house,
A high up in the leafy branches
Cozy as can be house.
A street house, a neat house,
Be sure and wipe your feet house
Is not my kind of house at all
Let's go live in a tree house.
~Shel Silverstein
