The city where intelligence warfare between North and South Korea is most intense

This is Shenyang, China.
With a population of 10 million, it is the largest city in Manchuria.
During the Goryeo Dynasty, it was where the largest number of Goryeo people lived under the Yuan Dynasty,
it was the capital of the Later Jin, and the place where Crown Prince Sohyeon lived as a hostage.
Because it is a major metropolis closest to the Korean Peninsula, it is highly significant,
and it has many Korean expatriates, ethnic Koreans from China (Joseonjok), North Korean defectors, and North Korean laborers.
As such, there are many intelligence agents from both the North and South,
and it is said to be a place where our agents engage in an invisible war.