Here are a couple of drawings from my plans for the fort. The rope bridge is not shown, but will go across between the two sections. Each of the pillars will be sunk a couple feet into the ground with concrete, and there will be ropes closing the gaps in the rails. The flooring is not shown, but will likely be treated 1x8 or similar. Also not shown is the ladder leading to the crows nest.
If you have any concerns or suggestions let me know, and if you'd like to play around with the model itself, let me know and I'll email it you. It's a google sketchup file; sketchup is a free tool, so you can just download it and use it to view the model.