In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The Oirat leader Esen Tayisi launched an invasion into Ming China in July 1449. The chief eunuch Wang Zhen encouraged the Zhengtong Emperor (r. 1435–49) to lead a force personally to face the Oirats after a recent Ming defeat; the emperor left the capital and put his half-brother Zhu Qiyu in charge of affairs as temporary regent. On 8 September, Esen routed Zhengtong's army, and Zhengtong was captured – an event known as the Tumu Crisis. The Oirats held the Zhengtong Emperor for ransom. However, this scheme was foiled once the emperor's younger brother assumed the throne under the era name Jingtai (r. 1449–57); the Oirats were also repelled once the Jingtai Emperor's confidant and defense minister Yu Qian (1398–1457) gained control of the Ming armed forces. Holding the Zhengtong Emperor in captivity was a useless bargaining chip for the Oirats as long as another sat on his throne, so they released him back into Ming China. The former emperor was placed under house arrest in the palace until the coup against the Jingtai Emperor in 1457 known as the "Wresting the Gate Incident". The former emperor retook the throne under the new era name Tianshun (r. 1457–64).
Tianshun proved to be a troubled time and Mongol forces within the Ming military structure continued to be problematic. On 7 August 1461, the Chinese general Cao Qin and his Ming troops of Mongol descent staged a coup against the Tianshun Emperor out of fear of being next on his purge-list of those who aided him in the Wresting the Gate Incident. Cao's rebel force managed to set fire to the western and eastern gates of the Imperial City (doused by rain during the battle) and killed several leading ministers before his forces were finally cornered and he was forced to commit suicide.While the Yongle Emperor had staged five major offensives north of the Great Wall against the Mongols and the Oirats, the constant threat of Oirat incursions prompted the Ming authorities to fortify the Great Wall from the late 15th century to the 16th century; nevertheless, John Fairbank notes that "it proved to be a futile military gesture but vividly expressed China's siege mentality." Yet the Great Wall was not meant to be a purely defensive fortification; its towers functioned rather as a series of lit beacons and signalling stations to allow rapid warning to friendly units of advancing enemy troops.
What is the name of the fortification whose towers functioned as a series of lit beacons and signalling stations to allow rapid warning to friendly units of advancing enemy troops?