I just thought about making this right before I finished building my fort so there might not be that many pictures, and its kind of messy. Well hope you enjoy it :)
Step 1: Finding the Spot
But a square tower would work also.
This instructible is for a triangular tree fort so if you want a square then think about how to make a square using this.
Step 2: Materials
Another person ( to hold up the base and support boards while you nail/ screw them in)
Saw ( I used a regular hand saw but any other saw would probably work to)
8/10 large foundation nails
1 or 2 big boxes of screws
2x4's for the base and support beam ( length and amount depend on the size of your tower)
Step 3: Building It Part 1 the Base
2. Now, once they are in properly do it again, but this time use the level to make sure that they are in the same spot
3. Repeat step two until you have they main outline of the square/ triangle.
4. This is where you use the ladder. You set the ladder up so that you can climb up to the frame easily.( unless, of course it is close to the ground)
5. Now you climb up the ladder and place down the first board. Make sure it is the right size before you screw it in.
6. Continue step five until you have the main platform that you can fully walk on.
TIME FOR PART TWO
Step 4: Building It Part 2 the Walls
2. Have your friend go on top of the platform and hold up the 2x4 while you screw it in with a two long screws on each end.
3. Do step two again, but on a different side.
4. Now, there should be two railings and one open side.
5. Now here is the tricky part. see drawing above. ( the dots are screws And the boards are 2x4s)
6. Next you move the ladder to the open gap between the tree and the half railing.
7. And voila you got a tree fort.