siege question

[Taqwus]

Gate Cleaver, Wall Shaker, Siege Golems come to mind. So does the Crumble spell -- that one HURTS (e.g. minimum 150 points of damage to the castle, IIRC).
Given that it's C'tis and your dominion, Purgatory might help; one possible explanation would be that the defenders have numerous undead and are reanimating more, in which case starvation is basically irrelevant.