Historically the weakness of the phalanx was the lack of mobility and fact that they have to walk, because once you get around their flanks they are screwed. You don't neccessarilly even need a centre. You can just attack with two flanking forces. That dirty great wall of spears is no use if it has nothing to smash into.

Ultimatly though if you are assaulting you've already lost. You lose a huge proportion of your army, usually less troops than the enemy loses and you've damaged the walls, so if you have to defend next turn you are trying to plug all those lovely breaches that you made.

a Siege is a waste of time and resources, more so than a pitched battle, but an assault is worse. So says Master Sun.