Treehouses and other boy-friendly structures

I remember the following treehouses or otherwise "natural" forts that people had when I was growing up:

• Jimmy's lean-to: It was uphill from "Snake Creek". As I recall, it was built on the crest of a hill, and was made out of birch logs (they were clean white logs) lashed together, with a log roof, and log sides. Someone had also built a stone fireplace so that whoever camped there could enjoy it. Nearby was an old 40s-era car resting upside down and rusting away.
• The complete underground fort: Near the swamp on the edge of the farmland was an underground fort. I recall a square opening that you could jump through and the area underneath was damp and muddy. It could have been the former "basement" to someone's house, but. . .to me, it was an underground fort.
• The "F..." club: somebody had built a treehouse and had labeled it the F... club. Never saw it in use for that or any other purpose.
• My own treehouse. . .in the Dead-end Woods. It was a few boards nailed to a tree, but I was able to climb up into it and watch things happening in the woods.
• The unfinished underground fort: Jimmy and I dug this fort out of the top of a creek bank, and it had an entrance that was supposed to open into the creek bank. We never finished the top (as in, laid logs across and put sod down). But it's still there for whoever happens to stumble upon it.
• The big oak tree in Mom and Dad's back yard had a three-way branch about 15-20 feet up. I remember nailing boards into the tree to serve as hand and foot holds. In the crotch of that branch, I had nailed a green painted square board. I recall climbing up to this perch and sitting and watching what was happening in the neighborhood.

Oh to have a treehouse again. This one's for you, Sarah.