Antikinesis said:
I sense a trap. 
The problem with using that scenario is that in D&D, doors usually provide 100% cover until they're open or destroyed. Nothing wrong with modifying that in a dungeon setup, though. As a door's hit points drop from being attacked, reduce the cover provided by the door.
If the fellowship had prepared ranged attacks for the first available target, they would get some shots in as the cover provided by the crumbling door is reduced.
Sound reasonable?
-AK
More of a Troll, but its also a good example, because its visual, was done well, and similar circumstances happen in D&D.
Well in the movie, not only did the Fellowship get a couple of shots off through arrow slit sized holes, they also got shots off right after the door collapsed and before the orcs were even able to attempt attacks.
Ok, so lets do this via D&D rules.
1) Orcs and Fellowship are aware of each other.
2) Roll Init.
3) Orcs use rules for attacking inanimate objects. Hardnes etc.
Fellowship readies actions.
Orcs in the back ready actions.
4) There are no rules for partial breaking of the door so the Fellowship does not get to make any attacks as holes open up in the door.
5) The door comes crashing down. Orcs in front row who were bashing the door don't have readied actions, they can't because they were using there actions to attack the door. But they can move in, albeit after everyones readied actions go.
Fellowship's readies actions trigger at the same time that orcs in back go. This is the gray area, who goes first? By the rules the Orcs can move through their comrads squares unimpeded. Maybe the fellowship is far enough back that they are out of partial charge range. But what if they aren't? Possible solutions:
- Highest dex goes first
- reroll init, possibly giving Fellowship and 2nd row orcs a decent circumstance bonus
- Readied missile attacks go first, in dex order, then readied partial charges, in dex order.
- Orcs need to make balance checks to get past their comrads and over a broken door, any who fail stop in there charge
- 2nd row orcs can't partial charge because there buddies and the broken door do not allow for straight movement
- some combination of the above.
Now if the Fellowship had not barred the door:
1) Orcs and Fellowship are aware of each other.
2) Roll Init.
3) Fellowship readies actions.
Orcs in the back ready actions.
Orc uses a MeA to Open the door. If he has a missile weapon or a fellowship member is within melee range he can attack.
4) The door opens.
Readied actions trigger, with the same complications above.
Orc door opener can now move in.