@Lord Stason - The only other advice based on my experience is to make sure the units using the siege towers have all disembarked the tower before trying to order them down the wall defenses or your pile will never go anywhere because a few stragglers were down on the ground waiting to climb up or still in the tower and climbed down the tower instead of proceeding up as originally order and will usually never find the main gate to join up with their unit. I never rely on siege tower units to reinforce my attacking army. I just use them to gain control of the main gate and plow the rest of my army through and capture the city. It usually takes too long for the units on top of the wall to get off the wall to be of any use taking the middle; its possible but not practical.
And as DKV said make sure to check your units 'fire at will' option, if it isn't on when you attack they won't throw unless you select your unit and select an enemy unit to attack. Having the option off is useful in some situations but not in what you described.