I noded about 20% of the ground, and double-noded (walked along edges, so as to fit two nodes) all the walkways, and triple-noded the ladders in the towers.
After about two hours of play, they have
never used the ladders. They
rarely use the ramps (is this because they think that it is too high up for some of the big ramps?) and due to my very little amount of ground noding, they only take a small path.