I have a personal theory that all playhouses and treehouses--forts are best built by dads and grandpas. Of course I've also had this urge to design and market playhouses in various states of completion for single mothers. I can't think of a better role model for a little girl than to know her mother could and just importantly, would, make her playhouse.
A couple of thoughts on the fort.
One, the one I built with my son ended up having a knotted one inch rope for access. I was a little nervous at first about having it eight feet off the ground but it worked out that was the perfect height. It was just high enough that they held on tight and didn't fall. I suspect lower would have encouraged more experimentation on the limits and effects of gravity.
A side note about the knotted rope. I found out that it turned out to be a good thing. It seems bigger kids naturally have a little bit more of a tendancy to bully than the smaller ones. My son was smaller. It also seems to be a fact of nature that bigger kids aren't as physically gifted when it comes to climbing ropes and smaller kids are. So it was a boost in my son's self image when he could be the one to excel at tree forting one oh one.
My son is now thirty. He still likes to run on and on about having the coolest fort ever. Funny thing about memories, they always seem to grow with time.
Congradulations. Let me pass on to your son what an older neighbor passed on to mine when he saw our fort. He said, "young man, one of these days you'll figure out that you're the luckiest boy ever, having a kid for a dad."