One of two things. Mostly, I walked into the quest and had to start scrolling through my journal to try to find out how to operate catapults.
So either put up a sign in the area just to ooc explain things to players, or if some sign is up make it easier to find.
Also when I went in, there were only two or three rounds of ammo in the barrels. We pretty much just fought the rats normally and fired off a few useless rounds over the rats heads.