If you don't have a tree to build with.... I would also put a screen and maybe a tarp over the opening for bugs and rain
Wooden Cabin & Scramble Climbing Frame
Tree House / Club House | eBay
This looks like it would take up about half of our yard...but I'd LOVE to do something like this for our kiddos!
Relaxshacks.com: A tiny house/study pod for an NYU Professor….on wheels
This would be so cool to have! I think the boys would never leave the backyard! now to convince T to build it!
A tree house platform over looking a field in Hampshire
Instead of a treehouse, build a DIY tree fort. Kids love multiple entrances and exits!
Garden Play - Tree Houses - Page 1 - Caledonia Play