Generally speaking low level archers will hardly reduce morale because they will hardly kill any unit with flaming arrows due to horrible accuracy. The distance between siege equipment and archer plays some role in the chance of setting it alight due to loss of accuracy over range. However, I have yet to see a human player's archers on the walls shoot down a siege tower that is relatively close but has not yet latched on.
That said, normal arrows have almost double the accuracy of flaming arrows, so you will kill at a higher rate with them especially since you got the height advantage. To use it, you have to place your archers as I described in my previous post by having them hug a tower away from the enemy in square formation.

Towers vs ladders. The dispute can be endless, but generally towers will reduce your casualties as opposed to ladders.
1. Even a burned down tower is a large obstacle covering the exact same dimensions as an intact tower. Therefore you can use them as cover for your troops.
2. As a tower approaches the wall, you generally hardly lose any of its carriers while you lose significant amounts of your ladder carriers.
3. Units disembarking from ladders suffer a morale penalty.
4. As you disembark from ladders, you get 2 at most 3 units on per 3-4 seconds. If the enemy on the walls has one hand weapons that are not spears, they will easily kill your ladder troops, inflicting heavy casualties as those do not have a chance to fight back at all (immobilized for 1-2 seconds on reaching wall)
5. As you disembark from towers on the other hand, you get about 20 men rushing down with the rest close behind. You can also overload a tower to about 30 before opening the hatch if you know how to. Those push away the enemy breaking their formation which gives you an advantage and even if they kill the first few coming out, the rest due to sheer numbers and mass actually get to fight back more quickly and will prevent the wall unit from escaping unless in a rout, allowing you to focus your efforts elsewhere.

The only time I would advise ladders is when you are trying to exploit the AI or if you are storming a citadel or if you are attacking from 3 sides and need to be quick.