Well, the boys got jealous. They cannabolised the girls playhouse and dragged the materials off to the trees. The boys enjoy climbing trees. So they built a grandiose staircase into the lower branches. This negated the need to climb, so they abandoned it in favour of of good old climbing. They did, however, fiercely defend any attempt to reclaim the materials they were not using.
Now, a boy cant lift a tire. But several girls working together can. And this tire stack technique can’t be kicked down. The girls organised Fort 1.0. With a few structural suggestions from an outside consultant ( me), they built a crenellated-reinforced curtain wall in the most defendable high ground.
Then they decorated it.