Over that terrain, I built the basic structure of the fort walls using Bryce primitives (cubes and cylinders). With more terrain objects, I tore out large chunks of the walls (upside down boolean operations). And, after giving it a basic texture, I rendered it from three angles - the top view (for the outside map), the side view (for the elevation), and the scenic view (for... um... the scenic view). I also rendered a bunch of masks for post-render tweaking.
The rest was just me in Photoshop with a Wacom tablet using the assorted techniques gleaned from this fine forum.