His life hung on the position of a single brick. The architectural engineer had meticulously calculated that he needed precisely 99,999 blocks to construct the tower at the Jiayuguan Great Wall's main pass. But his boss demanded he use exactly 100,000 — or die.

So, the sly engineer built according to his original plan but, when it was finished, he precariously perched a solitary block atop the tower's narrow ledge. When his commander saw this ostensibly leftover piece, he erupted into a tantrum and declared he'd execute the designer. But the shrewd builder insisted the seemingly useless brick actually braced the entire structure, which might disintegrate into 100,000 pieces if disturbed.

The leader felt incredulous but dared not disturb the brick or kill the engineer — just in case he wasn't bluffing. Otherwise, it might instead be his head on the chopping block.

A replica of this storied slab teeters atop the structure's edge today.

This fable is one of many myths and true tales written in stone at this Great Wall terminus.

Another account says that the bricks were hewn and transported from the Heishan Mountains. A clever old man suggested pouring water on the frozen ground to "pave" slick ice paths along which laborers could slide these hefty rocks to the building site.