Generally speaking, mountains marked the frontier, and the flat ground between them was where nomadic peoples invaded.
If the Great Wall were built only in those gaps, troops would have to be stationed at every one of them to keep watch. Without horses, their mobility was poor, and they could not readily reinforce other positions.
If enemies appeared on a western peak, troops on an eastern peak would have a long journey to intercept them, having to go around the mountains.
Smaller mountains were manageable, but great ranges like the Taihang Mountains and Wangwu Mountain were another matter.
Tang Sanzang and his disciples had been forced to brave the hellish difficulty of Lion Camel Ridge because the damn mountain was simply too big. Elder Tang could not go around it.
They had to wind their way up the mountain, then twist and turn their way back down.
That would not only render the beacon towers mere ornaments; by the time help arrived, the enemy would already have smashed through the Great Wall on the plains, plundered to their hearts' content, and fled.
But if the Great Wall were built across the towering mountains and precipitous ridges as well, things would be different.
The sections on the mountains would link up with those on the plains, forming a continuous road.
Then a single army patrolling the Great Wall could handle its defense. No sooner would Tu Fang, Gui Fang, or anyone else breach the wall than the defenders would arrive, using its fortifications to establish defenses and counterattack.
Of course, these were all long-term plans. Foolishness Value had to be pursued slowly and steadily.
Zi Shou's plan was to build earthen walls first, then use them as the foundations for the Great Wall.
They could use local materials, primarily rammed-earth walls