Treehouse improvement,,,,,,,,need more letters in title...?

well me and my brother have started a small treehouse in a pine tree that had some nicely placed branches. All we have so far is the floor put down and nailed in place. Its very small, only like 4ft by 4ft. we would like to add walls, a roof, and another room to it. Can someone give me tips on doing this? I was thinking to put down some 2 by 4s for the floor in the new room,and 2 by 4s also going up to the ceiling in the corners. just some big planks for the walls and roof. We are also a little short on wood (lol) but I have plenty of nails and a hammer. This was also just a thought, how hard would it be to put in a zipline going from the treehouse to the ground? not a huge one, because the tree is only about 15 feet tall.