So.. not quite sure if this is the right spot for it, and dunno how realistic the idea is. But Goblins and catapults, any silly thinking Goblin must have thought about it before. Loading mobs of goblins into catapults, then launching themselves onto the walls. Or into them. If they survive, they are on the walls and ready to do sneaky goblin things, like stabbing silly defenders in the gut, or trying to mob the lord of the castle.
Of course if they splat into the walls, Isuppose their biting and clawing at the stones or wood might damage it a little. Certainly splatter them nicely with screaming splattery gobby bodies.