Remember the view from atop a tree? You could survey the landscape like a monarch, escape your parents' watchful eye, and let your imagination run wild. Well, just because you've grown up, it doesn't mean you can't enjoy the sense of liberation a tree provides. Here is the most comprehensive guide ever to building your own castle in the air, be it a simple child's playhouse or an adult's cozy retreat with all the modern conveniences, including electricty...