This is coolbert:
As has been described in previous blog entries the Xiongnu and Han at loggerheads for the better part of several centuries. See previous blog entries regarding this conflict:
Conflict between the barbarian steppe horsemen and the civilized Chinese the latter using special combat measures and techniques quite effective.
"What would be a feasible contemporary defensive technique/tactic/strategy to deploy against an invading Mongol army?"
Think not only the Mongol but any of those steppe-dwelling horse-archers such as the Scythians, Huns, Magyar, etc. And Xiongnu.
MOBILE FORTIFICATIONS! WAR WAGONS!
Fight the enemy with weapons you have and he does not! Fight the enemy on your terms and not according to the wishes of the adversary!
"How to beat the nomads on the steppes in 3 easy steps"
* Step 1: "Get lots of sturdy wagons"
* Step 2: "Fill the wagons with the best ranged artillery of your era"
* Step 3: "Circle your wagons and fire away"
This sounds like the strategic defensive! Invade the territory of the enemy, occupy terrain conducive to the defensive, compel the enemy to attack. Fight a strictly defensive battle, weaken the foe, all the while husbanding your forces for a final counter-offensive!!
"with your own cavalry inside, ready to burst forth and mop up if needed"
[artillery of the period defined to include archers and crossbow men. Missile firing troops]
"1st century: the Han dynasty clashed with the world's largest empire, the nomadic Xiongnu confederacy"
"In this era where steel was just emerging to replace bronze, the Han were triumphant in warfare against other city dwelling peoples but the nomadic Xiongnu with their elusive horse archers was an entirely different kind of challenge requiring its own set of innovation"
"The Han had a strong cavalry force of their own, but how could they bring their infantry forces (which can be very vulnerable to fast cavalry archers and lancers) to bear against a foe born on horseback (the saddle was actually recently invented around this time too)?"
"The innovation was in adopting wagons that were normally used for carrying supplies into mobile fortifications . . . an archer on foot was able to use bows and crossbows of a greater size and range, able to fire accurately and reload quickly on firm earth while a horseback archer needs to take the galloping of his horse in mind. This combination of high firepower infantry behind resilient fortifications with swift cavalry of their own was devastating against the Xiongnu confederacy."
Necessity is the mother of invention and all that. And a proven winner as established from decades of warfare between the horse-riding steppe nomads and the Chinese.
War-winning measures, the Chinese adaptive and innovative. I might imagine having to surmount a long and arduous learning curve as well. But persistent and also employing "barbarian taming" methods as they understood the term!!