Has anyone built a treehouse?

[pinkhousesarebest]

We have about four big old pine trees in our front garden,all close together. I would love to turn them into a centre of interest for the D.cs,who are 6 and 4. I had thought of a treehouse.Anyone any ideas?

My kids got 100mts rope as Xmas present from gramps last year. Made rope swing, rope ladder, towing ropes for trikes, etc. Still have metres left......

My childhood treehouse was a piece of ply wedged in the fork of a beech tree.