Great Wall.
It was of utmost importance for any ancient regime.
In the Central Plains, the greatest enemy was the ferocious nomadic tribes from the grasslands.
Before the Great Wall was built, incursions by barbarian cavalry into the south to plunder were frequent every year.
And since these were small-scale cavalry raids with high mobility, they would typically raid and kill and then leave, leaving the defenders powerless.
Until the Great Wall arose, blocking the trampling of horse hooves.
The nomads would need to pay a huge cost if they wanted to raid.
They either had to siege or cross mountains and ridges on foot.
The former required a loss of lives, while the latter, even if loot was obtained, meant being pursued and killed without the boost of warhorses.
So, out of necessity, the originally scattered tribes of the grasslands also began to unite gradually, eventually forming several powerful alliances to collectively oppose the states of the Central Plains.
Furthermore,
