Well, i am aware that those runes do disarm the corresponding statues.
BUT. If i remember correctly if you come in from the outside and go down to the goblins without disarming the eagle, you get shot. So why are the goblins not hit by it?
My guess was that it was because they followed the halfling in and his key made the staues stop firing.
Otherwise that defens system is just there to annoy players who want to get the key to the chest...why would they guard that small island and not the grove entrance...ehhhh?!?
Because Goblins are sneaky little bastards. They snuck past the statues and they are in a "safe" area. They probably failed to find the front door that leads to the grove, and can't unlock the upper door that leads to where the Drow is. They decided to take a break, interrogate the prisoner and maybe have a snack and a drink.
