What structure was built to protect northern China from nomadic invasions?

The Great Wall of China was built and expanded to protect northern China from raids and invasions by groups from the Eurasian Steppe. It is not one continuous wall from one construction period, but a defensive system built over many centuries.

Fortifications later incorporated into the Great Wall began appearing as early as the 7th century BCE, when rival Chinese states defended their borders. After unifying China in 221 BCE, Qin Shi Huang ordered existing walls connected and extended to guard against incursions, especially from the north.

Later dynasties repaired or rebuilt sections for changing military needs. The Han, Northern dynasties, Sui, and Ming all contributed to the wider system. The famous brick-and-stone sections near Beijing were built mainly during the Ming dynasty, after the dynasty faced repeated Mongol attacks.

The wall worked alongside garrisons, watchtowers, beacon towers, passes, and military settlements. It could slow cavalry raids and control movement, but it was not an impenetrable barrier; armies crossed through gates, breached sections, or exploited political and military weaknesses.