This is a MC schematic of a large tree house my friend and I made. I think it's pretty cool, and I hope you like it!
The entrance may be a bit tricky to find, but just hunt around under the trees for a ladder... Even I have a tough time getting in! But that makes it the ideal fort... Right?

* Schematics may seem daunting, but if you install MCedit, it's no big deal! It's actually easy!