As far as the diagonal supports are concerned, I don't think we need rope. My idea was to joint one layer of longitudinal logs to the transverse logs above and below it by driving large dowels through holes in the overlap. We would then have diagonal props with bird-mouthed ends pushed up and under the longitudinal logs like a clothes prop on a washing line. A further peg driven through this joint would prevent it slipping sideways. At the bottom end the pole would sit into a socket hole in the ground with a long thin wedge driven in to tighten the whole thing up. If the strength of the ground is insufficient on its own, we use a trench with a big stone in it to act as a foundation plate. We don't need to worry about the tower lifting off the props since we have a ruddy great stone sitting on top of it, keeping it in place. Even when the stone is dropping, the vertical load on the tower remains large until the point at which the stone breaks contact with the tower and by that time who cares anyway?