You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
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: Nearing London, Oliver encounters Jack Dawkins, a pickpocket more commonly known by the nickname the "Artful Dodger", and his sidekick, a boy of a humorous nature named Charley Bates, but Oliver's innocent and trusting nature fails to see any dishonesty in their actions. The Dodger provides Oliver with a free meal and tells him of a gentleman in London who will "give him lodgings for nothing, and never ask for change". Grateful for the unexpected assistance, Oliver follows the Dodger to the "old gentleman's" residence. In this way Oliver unwittingly falls in with an infamous Jewish criminal known as Fagin, the gentleman of whom the Artful Dodger spoke. Ensnared, Oliver lives with Fagin and his gang of juvenile pickpockets in their lair at Saffron Hill for some time, unaware of their criminal occupations. He believes they make wallets and handkerchiefs.
Solution: Who believes Fagin's gang make wallets and handkerchiefs?.
Why? This question is based on the following sentence in the passage "He believes they make wallets and handkerchiefs". It evaluates the understanding that the pronoun "he" refers to name "Oliver". You can ask questions like this one about most pronouns in a paragraph.

New input: 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.
Solution:
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?