Building a treehouse - advice needed

Got permission from the wife and found the perfect oak in our backyard. My dad says he'll help. Planning to keep it simple, 12 feet up, basic platform with railings and a rope ladder. Safety is priority one. Has anyone done this? Trying to figure out the best way to secure the beams without killing the tree. Don't want to spend a ton but want it solid enough for 3-4 kids.