@chendu They don't have to poke their heads out though? Read my comment again: "Uhh, can't arrows arc? Why didn't they shoot up? "
They should still be able to shoot arrows while in the trench, without looking. The accuracy would just be near 0, but if enough arrows rain, some would hit regardless. -- I think the author forgot about this because trenches were introduced with firearms, thus arcing shots were 'out-of-style' (aka no effect because bullets are meant to fly; lightweight). Then again, arrow rain would be that effective either, because projectile weapon deals damage due to acceleration, and the arrows would first have to lose the acceleration, to fall, though they gain some due to gravity, it would need to be really high, to do enough, unless the lucky hit (as mentioned before).
Also, speaking of, how come they weren't attacking the flamethrower guys as well? They were basically setting up what looked like a cannon for crying out loud.