AJ........What you are wanting to do is not very hard. Go get some 14' treated 4x4 posts and set them 3' in the ground and pour quickcrete around them. Let them sit for a couple of days, and make sure they are plumb and square. Add some cross bracing and your ready for step 3....

Next it's just like building a treehouse, just measure up and wrap the posts with 2x6's around the outside. Next cut 2x6's for your floor and lay across and nail to the 2x6's. You may want to add a couple of 2x6 joist in the middle every 2' if it a larger platform. Build a ladder off of one the post corners, and you have a platform. Next just build some frame wall with 2x4's and plywood. Even add a roof if you want.