Look at the picture and guess idiom 4. What is a door with a shield?

Pinyin: gān chéng zhéJiang Jiang

Source: The Book of Songs Nan Zhou Rabbit: "Correcting Beowulf, Gonghou Gancheng." Kong Juwei: "Now that you are in the Warring States, you choose your slave, but you abandon General Gancheng with two eggs. This is something that neighboring countries must never hear. "

Interpretation: Gancheng: shield and city wall, compared with defenders. Refers to the general who defends the country.

Example: At this time, Qin marched into Zhao, and Zhao had no generals to attack the city, and he was defeated repeatedly.