Magonels..2-3 of these in the center of your city have the potential to route an army
you of course leave a unit or two at the gates to damage as much troops as possible(retreat em when the ram is about to begin breakin thru the gates)...then make your stand in the center
when they get good hits they can kill 20-30 soldiers(this is on normal setting) and when engaged with your troops and u fire a few barrels it kills their morale(THEIR SHOOTING INTO THEIR OWN TROOPS?! RETREAT!)
my friend and i have come up with the ultimate defense:
1-2(3) Magonels - for smaller cities i wouldn't recommend getting 3
1-2 Mortors(potential to destroy siege equipment..if u dont have any in ur arsenal get catapults)
1-2 greek flamethrowers
1-2 archers
rest of your army mainly spearmen(they can hold the line no matter what for the most part) and some other type of infantry and calv
with the flame throwers try to have it so the enemy is engaged with your troops at an angle(have like 2-3 unites of spearmen here) so they can fire into them..kinda like this:
*=flame throwers
_ + /= infantry
* ______________________/
------------catapults catapults
---------archers/crossbows ----------------------------------- reserve infantry
-----magonel magonel
Calvary
if done right the enemy should engage your troops instead of going after the flame throwers(it may take you a few tries to get the positioning right)