Where do the Sixteen Prefectures of Yanyun refer to?

The Sixteen Prefectures of Yanyun specifically refer to: Youzhou (today’s urban area of ​​Beijing), Shunzhou (today’s Shunyi District of Beijing), Ruzhou (today’s Yanqing District of Beijing), Tanzhou (today’s Miyun District of Beijing), Jizhou (today's Jizhou District, Jin City), Zhuozhou (today's Zhuozhou, Hebei Province), Yingzhou (today's Hejian City, Hebei Province), Minzhou (today's north of Renqiu City, Hebei Province), Xinzhou (today's Zhuozhou, Zhangjiakou City, Hebei Province) Lu County), Guizhou (now Huailai County, Zhangjiakou City, Hebei Province), Wuzhou (now Xuanhua District, Zhangjiakou City, Hebei Province), Yuzhou (now Yuxian County, Zhangjiakou City, Hebei Province), Yingzhou (now Yingxian County, Shanxi Province) , Huanzhou (now east of Shuozhou City, Shanxi Province), Shuozhou (now Shuozhou City, Shanxi Province), Yunzhou (now Yunzhou District, Datong City, Shanxi Province).

　　Defense value

　　The area under the jurisdiction of Yanyun Sixteen Prefectures is about 600 kilometers from east to west and about 200 kilometers from south to south, covering an area of ​​about 120,000 square kilometers. Its northern part is in the northern part of present-day Hebei Province, at the northern end of the vast and flat North China Plain, including the majestic and steep Yanshan Mountains; its western part is in the northern part of present-day Shanxi Province, which is mountainous and has complex terrain. The situation in this long and narrow area of ​​Yanyun Sixteen Prefectures is very dangerous, and it was indeed a natural military danger in ancient times.

　　The Yanshan Mountains and the North Taihang Mountains lie in the northern part of the Yanyun area. In ancient times, when productivity was low, transportation was backward, and military technology was underdeveloped, mountains were an invincible natural danger in front of people. From the topographic point of view, the Yanyun Sixteenth Prefecture, the Yanshan Mountains and the North Taihang Mountains are like two connected city walls standing in the northern part of the North China Plain. The intersection of the two mountains is more like a "city corner" cast in the sky. The northern part of the country is dominated by cavalry. The nomads were severely isolated here. The majestic and steep mountains have formed the Yanyun area's natural first line of defense strategically against the northern cavalry moving south.

　　Sanggan River and Juma River run in the central and southern parts of Yanyun area, which are rivers in the strategic defense center. "In ancient China, rivers and swamps, especially rivers, often served as defensive barriers to keep out enemies. In fact, in the cold weapon era when swords, guns, bows and arrows were the main weapons, using rivers for fortification was a common way to defend against enemies. "In addition, rivers also Military strategic materials can be transported. The Juma River, with Yishui and Baigou rivers as its two wings, forms the river defense system in the southern part of the Yanyun area. The intersecting rivers obviously formed the Yanyun area's natural second line of defense strategically against the northern cavalry moving south.

　　The strict defense system in the Yanyun area includes not only natural barriers but also man-made defense lines, which are the Great Wall and the Five Passes.长城自秦朝开始修筑。这是中国古代一项伟大的防御工程。在漫长的历史时期发挥了巨大的国防功能。同时，长城也是农耕文明和游牧文明的分界线，发挥着保护先进农耕文明不被游牧民族无休无止的袭击所毁灭的历史作用。

　　尤其是燕云地区的长城，对于阻挡北方铁骑入侵中原王朝起到了巨大的作用。此外，该地区“五关”在战略防御中的作用也不容小觑。太平间，以五关为胜之形。这五关分别是金坡关、居庸关、古北口、松亭关、玉关。雄伟险峻的长城和五关口，构成了燕云地区抵御北方骑兵南下的坚不可摧的人工防线。