I built 2 this season. Used 2x4 for the ladders, platforms roughly 3 ft square. I angled the ladders out a bit to make climbing a little easier. Used strapping (1x3) for the rungs. Ran x-shaped braces across back of ladders and angle braces between ladders and the platform. Mine are 16 and 18 ft high. I pu a bench style seat on one, and use a strap on tree- seat on the other.

I know mine are stout, but your luck may vary, depending on your skill and the materials you use.