Just make sure you bury enough 4x4 in the ground to ensure it secure.
I have never built a wooden stand to 12'. I think build the box and getting the roof built might be interesting.I mean 12' to the floor would put the roof at say 18' if your inside is 6' from floor to roof. need a tall ladder. on steel ones i build i make the roof where it bolts on to the stand . i make attach points for the roof to be bolted prior to raising. much easier.


If i was going to build a wooden tower that height i would sure make it 4x5 it is really nice to have the room and you can have someone sit with you especially a kid.