Bug report:
Archers are critically bugged in your mod. It seems after restarting this siege 7 times trying to see what's going on: It's as if once an enemy approach of about 10-30 meters from the archers, almost all archers switch to melee or refuse to use their bows. Cross bows are for the most part fine, they shoot like normally, but archers are broken.
Fairly similar thing happening in field battles, but since sieges are inherently more close quarter the effect is drastically more noticeable.
Save game of 1.1 beta just before my first siege defense: https://drive.google.com/file/d/0B6baFjETxO8_MF9BRy0wWmJpdkU/view?usp=sharing
Pictures ingame. Notice: Observe the "combat text": archers use of melee weapon, even tough I give archers command to fire at will. A few archers who are very far from combat some times use their bow, other times they don't.
http://imgur.com/dGnQzzz
http://imgur.com/a/mTzlM
Other mods, you'll see archers consistently fire, pressing up against each other on the battlement to get their arrows out, but here they are completely docile and do nothing. I think it has to do with proximity, but I can't say for sure.
Hope this helps.
EDIT: Swapped out all my master archers for crossbowmen of similar rank and number of men. http://imgur.com/a/Kftnx Complete night and day difference.