In a fascinating article in the Atlantic Monthly this month (April 2014) entitled “Hey Parents, Leave those Kids Alone,” Hanna Rosin quotes sociologist Roger Hart, who studied children’s play. Describing his research on the movements of children in the 70s, Hart writes, “I was struck by the amount of time children spend modifying the landscape in order to make places for themselves and for their play.”
I am fascinated by children’s impulse to modify space and create worlds and forts and nests. I did (I had a secret place I’m sure my mother knew about behind the fireplace chimney, and at Margie’s house there were passageways between rooms and leading into a playroom/slumber party den). My children did–The Hobbit Holes at one house and Fort Danico at the other. My grandchildren (who have been architecturally deprived) do NOT, not at all (although Aleyna can turn any piece of trash into a Barbie costume). Come to think of it, I have a dog who is perpetually concerned with landscape architecture, so it may not be a human phenomenon.
orgatecture.org 
I assume you’ve read Roxaboxen? My kids were always constructing imaginative lands that I wasn’t supposed to know about.
NOW I do–thanks! Bridge to Terrabithia, Dinotopia, and Swiss Family Robinson feature fondly in our family lore.
So, Art added: I believe it was Picasso who said, “All children are artists. The problem is how to remain an artist once we grow up.”
Roxaboxen (the marvelous book) came today (that was quick!). These kids are like mine were, secure in their “special place.”
With freedom to create, they made a “world” they could master: “And so it went. The seasons always changed, and the years went by. Roxaboxen [Fort Danico] was always there.”