When you dreamed of creating a tree house, how did you imagine it? Maybe it shouldn't be very different from the adults' houses, maybe better and of course a secret entrance.
When I visited my grandparents in the Caribbean, their yard had hundreds of trees, some giant trees, others that I could climb, and there was one in particular that always seemed perfect to build my tree house. In the end, the result was something different from what I imagined (and planned), but even today I remember it as the best house I've lived in.