@Nunally Volley fire should be easy to get, but countermarch or platoon fire (two varieties of dividing your unit up in sub-units and having them in different stages of loading and firing, I can't recall the official name right now either) takes a bloody lot of drilling, since they need to be able to keep similar speeds of reloading and aiming while being in combat.
However something I didn't bring up which is similarly helpful is - if you got more people than wall space to fire from (and have the extra guns), you can have people less trained in firing handle the reloading. (Even the reloaders don't have to be well trained, as long as they are divided into groups headed by someone who knows what to do and can supervise/tutor them while they do effective work.
That way even normal citizens can make the half-trained gunners into a lot more deadly combatants. XD
(But if he really wants effect, instead of 500+ barrels of muskets, he should make like five pieces of artillery. Round balls skipping across the ground will cripple dozens of people per shot, and their screams will mean even more people will have to carry them away, or morale will suffer even worse. And the option for grapeshot or canister will really make the attackers think they're facing battlefield magic.)