Night battles

[katank]

Further, missiles don't work well in the night/rain. It can be advantageous to fight at night when facing a missile heavy army or doing siege assault. If you can get rain as well, then that's even better.

[TheManWhoWasn'tThere]

The biggest advantage I have seen so far in field battles is, that the enemy does not seem to see what you're doing. This way you can surround them easily. Or you can get your whole army around your enemy and attack him from behind, while his army is still facing in the wrong direction.

[Tuidjy]

You don't let the enemy bring his reinforcements, and defeat him piecemeal.
You can choose not to bring your reinforcements.
You get a one star bonus if your adversary is not a night fighter.
As you keep attacking at night, your bonus goes up to at least four stars.
You get less tired in the Holy hands.
Your enemy reacts slowly and poorly to your moves.
The missiles' range and maybe even accuracy are reduced.

Do you need more reasons?