Now correct me if I'm wrong but whole point of a night battle is to catch your enemy off-guard and in the ancient world it was the same as today. Marines, Seals etc.. don't go fight the enemy at night because they love the cool air and not being able to see , its because its when the enemy is expecting to be able to relax and eat, and warm themselves to sleep. So why in the is the enemy alerted to my attack (a garrison in my title but really this goes for all night battles) the moment I make it. It should be akin to a ambush, however the defending army should probably be made aware of your presence and where you deploy but maybe give him no time to deploy his and his troops form where they are(like arcade mode, remember that feature?) but for damn sure a enemy should not sally forth from a town to attack your army that is sneaking up to the town to attack you, that is just the dumbest thing I ever heard.
two legionaries: "Romulus be quiet, we don't want the Gauls to wake, and make sure you look out for anyone taking a piss from too much wine so we can kill him quietly"
"Romulus: There's no need centurion. Caesar told the enemy we were attacking at night and instead of getting drunk they are standing in formation ready to kill us."
I guess CA never thought anyone would care, but damnit its just another one of those things, that as a military person myself and as a wannabe historian that sticks out like a sore thumb waiting to be cut off