Q: This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
Passage: The first emperor of the Ming dynasty, Zhu Yuanzhang (the Hongwu Emperor), who overthrew the Yuan dynasty, renamed the city Yingtian, rebuilt it, and made it the dynastic capital in 1368. He constructed a 48 km (30 mi) long city wall around Yingtian, as well as a new Ming Palace complex, and government halls. It took 200,000 laborers 21 years to finish the project. The present-day City Wall of Nanjing was mainly built during that time and today it remains in good condition and has been well preserved. It is among the longest surviving city walls in China. The Jianwen Emperor ruled from 1398 to 1402. Question: What did Zhu Yuanzhang build around the city of Nanjing?
A:
long city wall