This is one I built for my kids. I built this about 1990 for my daughter (then 7) and son (then 4 -- he is being a zombie on the rope). The picture is from 1993 when he had a bunch of friends over.
It has a trapdoor you reach through the chain of tires (the tires slow 'em down when they tumble thru)...a slide thru the side (with a latchable door), a pulley on a lateral board to lift in big buckets of sleeping bags, tater chips, and other necessary victuals.
All windows were netted except the front side...the safety net was for the inevitable fall. They never admitted anyone fell into the net...probably because I let them know that getting netted was the least of their worries.
We had flower planters on the front with a gutter above and a funnel from that to water the plants with rain off the tin roof...oh, and a hammock inside for ME.
My daughter remembers sleeping in it during rainstorms and hearing the rain on the tinroof...she also got mad at us once and moved into the treehouse for about a week...took big plastic bins of food, clothes and personal stuff (books, lights, dishes from a flea market, etc.)
When the wind blows it still creaks like an old sailing ship.
The TH is still there, needs some reworking...Alex now has a 5 month old...we plan to get it ready for him.
