[Kobal2]

I think they just crap onto the besiegers from high altitude, lowering overall morale and efficiency because honestly, who likes catching a fat dollop of sky yogurt smack in the face ? Even the undead care about that sort of thing. The prospect of having one's soul ripped apart and cast into oblivion is one thing, but guano ?! A man has to draw the line somewhere.

When they do it on the offensive however, it's just more slippery material on top of the walls !