After you have talked to Captain he will explain that the goblins now have stolen and are operating cannons, which are defeating the dwarfs and that you need to put a stop to it. He then asks you to go and retrieve one of the cannons from their hideout. Go to the hideout and search around until you find a door. Make a odd potion with the Guam leaf and eye of newt and use it on the door. You may want to take a few steps back now as the door will explode, hitting you for 21 if you get to close.