Instead of a tree house, my dad built a fort in our back yard. It started as an open-air platform raised on four posts and shaded by a tarp roof. As amazing as it was, we didn't stop there. The fort received a shingle roof, walls with windows, a metal kitchen cabinet, a front porch with a slide, and a swing underneath.
Minus the slide and swing, I'm looking forward to watching the same evolution here. Already, the tarp has come off with better weather, and the rafter tails are going up.