Back to back: back. Fight to the death with the enemy at your own doorstep. Refers to the final battle that decides life and death.
General Gancheng Gancheng: Shields and city walls are metaphors of defenders. Refers to the general who defends the country.
Gaocheng deep pool: moat. The city walls are high and the moat is deep. Describe the defense as strong.
Attack the city slightly: plunder. Attacking cities and plundering land.
Siege city: city. Attack the city and fight wild.
Guancheng, Mao Ying and Mao Ying are all synonymous with pen.