They have gate for god's sake, if you know where they are heading why not someone inform the king first so he can evacuate the whole population and make a defensive arrangement, so even the devil reach the city you can minimize the casualties, you can use the gate to fasten the evacuation too.
Ps: Its pretty logical for Shin to be stupid in war tactic because he is a boy that been raised in a forest, but August is there, first prince, crown prince, and surely under tutolage by the best educators in the kingdom. Why he not the one who take command? Surely in a world where a godly being life side by side, Earshide will have a manual in case of impeding doom.