Using Autodesk Mudbox I've been trying to sculpt a rock wall from a simple base mesh in Maya which will hold up in quality in the animation. The first resolved attempt is below, which should be a good starting point. We'll also need a large rock for the centre of the town which will come next. There is sure to be improvements and I expect a complete new model for the rock as better techniques are worked out, but it should be a good starting point to place our town alongside. I think maybe more references pictures are needed and a more refined colour texture worked out (Maybe even a bigger resolution map) but the using normal maps, the render times are quite short.
One trick I used was blending two normal maps together, one being the Mudbox sculpt and the other a normal map converted from the diffuse. The quality seemed to improve as the overall shape was derived from the Mudbox sculpt and the smaller details, such as fissures, from the colour normal.