Here is a diagram which shows a cross-section of the raised road upon which Carnelian and Osidian travel in chapter three of The Standing Dead. Note how high it is above the ground. Note also the ramp which runs down to the ground on the right, besides which I drew a human figure to show the scale.
One of the several watch-towers they encounter is shown rising from behind the leftway wall which runs alongside the road, beyond a ditch. A view of the tower as seen from the road is also shown