Having unified China under his rule in 221 BCE, Qin Shi Huang faced the Xiongnu, a well-organized nomadic confederation on the northern steppe. The Xiongnu's cavalry raids continually threatened the settled agricultural zones. The emperor assigned to general Meng Tian and a large army the task of linking the separate northern walls that earlier states — Yan, Zhao, and Qin itself — had built along their own frontiers.

The wall as built was not the masonry structure imagined today. Surviving Qin-era segments are defensive lines of rammed earth (hangtu — 'pounded earth') drawn from the local soil, reinforced in places with rubble stone and wooden framing. The line was not a single straight wall either: it was a discontinuous system following rivers, valleys and ridges, supported by towers, garrison gates, and signal posts. Traditional figures put the length at around 5,000 li (~2,500 km), but these come from later sources and should be taken cautiously.

The human cost was heavy. Han-dynasty historians describe hundreds of thousands of peasants and convicts conscripted to the work, with many dying on site — giving rise to the legend that the wall was 'built of bones'. How much of this is history and how much is later commentary aimed at underlining Qin's harshness is still debated, but field archaeology has confirmed mass graves along the line. The construction was among the social burdens that hastened the dynasty's collapse.

The Qin wall did not permanently stop the Xiongnu — the succeeding Han dynasty extended it and then complemented it with military diplomacy and marriage alliances. But the model it established — marking the northern frontier with a continuous defensive infrastructure — set the reference for every dynasty that followed. Almost all of the stone and brick wall that today's visitors see is far later, the work of the Ming dynasty (14th–17th centuries); but the route that Ming wall follows is largely the line that Qin first drew. The 'Great Wall' is not a single structure but the accumulated layers of a two-thousand-year state project — and the first of those layers is Qin's.