QUOTE (jhf0551 @ Sep 14 2008, 10:45 AM)

For the Great Wall, I mean Ming Great Wall.
Since then, Ming still built several forts in Hetao, they were called "Duoyan Guards".
The Great Wall around Beijing was built in Hongwu's Reign, but those in the western provinces were built much later. The Great Wall in Gansu was even built in the 1570s, which is nearly the end of Ming dynasty. That indicates Ming's control over western Inner Mongolia before its decline.
I think it is possibly you made an error in defining "Duoyan Guards"; it is not a fort, and furthermore it is not in Hetao.
It is better to translate it as "Duoyan Wei"(朵顏衛), Here "wei" is a kind of administrative unit. Another type of administrative unit is province (in Ming it is called Buzhengsi布政司 but we can use the term province to call it).
The difference between "wei" and province is that "wei" is a military-dominated unit.
For example, the Guangdong province was called Guangdong Buzhengsi (廣東布政司)in Ming dynasty, while Ningxia was called Ningxia Wei (寧夏衛) since it is located on the northern border with the nomadic.
Now talking about the "Duoyan Wei", it is a Mongolian tribe living the west-Liaoning, just to the north of Beijing. There are several Mongolian tribes who are loyal to the Ming government, "Duoyan wei" is one of them. This Mongolian tribe is one important force in protecting the northern border of Ming; and it was also the main military force that the Yongle emperor Zhudi had before he usurped the throne and became the emperor.
Notably the Manchu tribe, before its rebellion against the Ming, was called the "Jianzhou Wei" (建州衛).
Furthermore, I think that the Great Wall should not be confused with the border. The wall is a defensive devise; it was built for military purposes. but it does not mean that the country who built the wall does not have control over the regions outside the wall.
From the Ming's side, it has two options during a war: either to fight outside the wall, or to close the wall and defend on the wall; of course the latter is the last choice.