After attending recent conferences and got in tone with the newest archaeology, I would like to make a few comments on our newest understanding of the Great Wall and Han northern fortifications.
Another thing related to this topic is the issue of maps. This is the best map I could find for the Great Wall with a cursory browsing of the internet and yet it still has problems. The northernmost part of what this map labels as the Han wall was actually built under the control of the Tanguts and not the Chinese. "Lost" Great Wall of China Segment Found?
The problem with most of these maps is that they simply lack precision, it would be nice to have a map that facilitates zooming in like that of google earth. This way one could see how the wall was built along the lay of the land.
Another point about these maps is that many walls are claimed to have been built on top of older walls. One example is the argument that Ming walls are reportedly built on Northern Qi walls, but how do we really know this is true? Is there physical evidence to back up these assertions or does the claim rest on textual interpretations?
First, yes, Kovalev's expedition showed that the furthest northern walls which was labeled as Han in the northwest were actually Xixia walls. The walls in Mongolia are Liao walls, not Jin walls.
However, in 2009, we have uncovered the exact location of the Shouxiang fortress of the Han built in 105 BC. Long thought to be in Inner Mongolia, this fort is now identified to be in Bayan Bulag of the Southern Gobi Province of Mongolia (in square on the map) even further north than the Xixia walls.
The newest excavation from Lin Meicun this year identified several forts in the Tuv Aimag on the Kherlen river, long thought to be Xiongnu cities, to actually be Han forts created in 102 BC by Xu Ziwei; this includes the long excavated city of Tereljin. The forts Xu Ziwei built reached as far as Lugou, and this Lugou mentioned in the Hanshu and Shiji was in fact the Kherlen river; 汉使光禄勋徐自为出五原塞数百里，远者千里，筑城障列亭至卢朐
The Outer forts 外城 which was abolished in 67 BC in the Han records weren't actually the Guanlusai Great Wall (or northern Han Great Wall), they were these independent Han fortresses deep north of the Gobi going as far as the Kherlen river in central-northern Mongolia.
We know for a fact from Hanshu that the Guanlusai Great Wall was still there in 51 BC and Huhanye settled outside of it. The Shouxiang fortress was also still around then, and probably remained so until the end of the Western Han.
This show that the northern frontier of the Western Han was much further north than previously thought; with the northeastern frontier reaching the Kherlen River, at least until 67 BC (then withdrew to Shouxiang fortress, still north of the Gobi), covering a good southern half of today's Mongolia.