What were the Inner and Outer walls built during the Ming dynasty?

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Great Wall of China Great Wall of China, near Beijing.

During the Ming dynasty, the Great Wall was extensively maintained and strengthened to defend against potential Mongolian invasions. Starting west of the Juyong Pass, the wall was divided into two distinct lines known as the Inner and Outer walls.1

These lines were supported by strategic fortresses and gates, which were critical for protecting the capital, Beijing:1

Three Inner Passes: This group included the Juyong, Daoma, and Zijing passes, which were the closest to the capital.1
Three Outer Passes: Located farther west, this group consisted of the Yanmen, Ningwu, and Piantou passes.1

Both the Inner and Outer passes were of vital importance to the defense of the empire and were typically heavily garrisoned.1