I wonder why some are finding the Hornburg impossible to assault, while others are able to assault Minas Anor (at least in terms of getting towers up there)...
The AI is fairly terrible at siege assaults, so I wouldn't judge the strength of the towers based on the AI's inability to mount a successful assault. If an experienced player is finding it impossible to assault, though, that is worthy of another look.
Were you at a bad point in the walls, perhaps? (i.e., lots of tower coverage hitting your siege gear?)