铜山铁壁

Explanation: Describe the strong defense material. Strong analogy reliable figures. 　　Source: "History of the Song Libo Yu Biography" Zhaoru Teng eight persons recommended to try, each item in the Boyu said 'Tongshan iron wall. 'Stand towards the wind section, large more like it. " 　　Examples: You teach your heart, such as stone also afraid of fear, there was ~ no use. (Yuan Shang Zhongxian "Liu Yi Shu" second off)