No.. it can't be.

[Terraforce]

Actually I had 2 units of Persian archers and 2 units of a new kind I decider to try out. I had 4 units of skirmishers. 4 phalanx, some light infantry the rest cav. The thing is is that they had so many archers and slingers that a good 70% of the front wall had some kind of ranged unit on it. Example I had a unit of skirmishers with ladders. By the time I finally reached the walls I had 64 men left out of a unit of about 210 it was. I did manage to get one unit of phalanx on the walls only to be promptly swarmed and destroyed. Honestly most of my men did not even make it through the gates. One phalanx did, it went through the hole in the wall made by the sap point, only to be charged by a unit of elephants while forming up. On a good note I did manage to kill the general.

[Aemilius Paulus]

Actually I had 2 units of Persian archers and 2 units of a new kind I decider to try out. I had 4 units of skirmishers. 4 phalanx, some light infantry the rest cav. The thing is is that they had so many archers and slingers that a good 70% of the front wall had some kind of ranged unit on it. Example I had a unit of skirmishers with ladders. By the time I finally reached the walls I had 64 men left out of a unit of about 210 it was. I did manage to get one unit of phalanx on the walls only to be promptly swarmed and destroyed. Honestly most of my men did not even make it through the gates. One phalanx did, it went through the hole in the wall made by the sap point, only to be charged by a unit of elephants while forming up. On a good note I did manage to kill the general.

You should never engage in combat on a wall. You will always be at a disadvantage there. Especially if it is phalanx you are using on a wall. Phalanx's main purpose is to engage the enemy with its pikes, keeping the enemy a sufficient distance away with the long sarissas, not to battle in tight spaces with shortswords.

You should attack a walled town with couple of groups of your units attacking the town at different sides but at the same time. Have three diversionary groups and one main one. The diversionary groups could be as little and weak as a unit of levy spearmen or skirmishers, just as long as it draws the AI units to away. Try to get the main force to climb an unguarded section of the wall with ladders, capture a gateway, and let the other main units swiftly rush in and take their positions. After this you should have no trouble with the AI. You can also put some missiles on the walls, for extra effect. Archers and especially slingers are real killers when they're shooting from the walls.