Stretching over 600km from Khentii aimag to China, and through all of Dornod, are the ruins of the Wall of Chinggis Khaan. This is not promoted by Mongolian tourist authorities because it was not built, or used, by Chinggis Khaan, but almost certainly created by the Manchu to limit (unsuccessfully) frequent raids from rampaging Mongolian hordes. Locals know it as the Chinggisiin Zam, or Chinggis' Road.
You will need a guide and jeep to find what little remains from the ravages of vandals and time, though it's doubtful whether it's worth the effort. The best place to start looking is about two-thirds along the northern road from Choibalsan to the Russian border, near the village of Gurvanzagal (also known as Slimiin Bulag).