I'm working on an article about this piece for FigureMentors in which I'll describe in detail how I approached the piece and attempted to let the character's story guide me. In that I'll discuss how I built the base, but I wanted to share a few pictures of the process here...
After coming up with the concept, I roughed out the shape of the base using cork. The cork is torn, not cut, to create the rocky shapes. I used some sculpting tools to break off pieces too, carving out the recess in the front. The figure was set on the base at this time to see how it all looked in relation.
I then began constructing the bridge. I started with the main posts and support ropes.
From there, I created the planks from balsa wood. I used an x-acto knife to carve up the edges and make them look more like old wood planks.
With the bridge finished, I started to flush out the ground details. I used a modeling compound (a bag of powder stuff you mix with water from the art store) to smooth out the soil surface. At this point I also added a number of real stones. That way they were submerged in the soil and not simply resting on top. The modeling compound doesn't have a lot of grip, so when it had dried, I applied a layer of 'Super Thin' superglue over it. This helped to lock them into place, without adding thickness (obscuring detail) like a gap filling superglue might. That is the shiny look you can see in the picture.
To provide some further texture, I used sand. Taking some white glue and an old brush, I applied glue to various areas on the top and sides of the base. I then sprinkled sand over these. This gives more of a rocky texture (the actual rocks are more like large stones, while the sand are small rocks) and helps to hide the look of the bare cork on the sides. When dry, I used more of the super thin superglue to help hold the sand in place (as it can be knocked off more easily if only white glue is used).
The base was then primed and layers of brown were applied. This is still early in the painting stages, but you can see how all the above materials look with some color.
Lots more painting followed, along with the final addition of grass, arrows, and of course the figure.