I'm making a timeline for my main story's world and simultaneously updating the "ancient" version of the political map. As I suddenly decided, my timeline called for a wall to be built. So I had a wonder.

Does anyone have any thoughts on things to keep in mind when mapping walls? And not just any little wall. I'm talking big time; Great Wall of China-type walls. Obviously, my two cents here would be to consider such things as natural barriers. Building between two difficult-to-pass mountain ranges is much easier than building across a huge plain to the north of those ranges and much, MUCH easier than building OVER the mountains.

So what are some other ideas? I'd like to hear your thoughts on the matter. (And if there's already a thread that discusses this topic, do direct me and anyone else who reads this thread to it!)