Why were the Great Wall passes near Beijing heavily garrisoned during the Ming dynasty?

During the Ming dynasty, the Great Wall passes near Beijing—specifically the Juyong, Daoma, and Zijing passes, known as the Three Inner Passes—were heavily garrisoned because they were of key importance in protecting the capital from potential Mongolian invasions.1

The Ming rulers ceaselessly maintained and strengthened these fortifications to prevent such incursions. These passes served as major strategic strongholds, functioning as command posts and access points for patrols or counterattacks against raiders. To manage these defenses, the Ming military organized the wall into nine defense zones, or "Nine Border Garrisons," each overseen by a post chief (zongbingguan), ensuring that critical entry points near the capital remained under constant watch and military control.123