I did a map like this a bit back.. Don't have the psd file though anymore.
But what I did (I think anyway..). Is for the top (and maybe middle) elevation layer I put on alot of layer styles and such.. And played around with it. The end result was satisfying. And just roughed up a bit of the ground.
http://www.cartographersguild.com/sh...wn-Of-Sendarai

There's also a couple tutorials on the forums about walls and pallisades, Just gotta look
(If you can't find any for pallisades what you could do.. (I think it would work anyway.) Is a hard round brush (size depends on your scale) With a spacing of like.. 100 or something with layer styles added onto it and a color overlay of brown. That's all I can think of right now.