Unfortunately, while most of the plan went well - they seem to have noticed the damage from the disintegrate spells, and had moved their forces out of the damaged wall sections, aside from some engineers that were attempting to patch up the damage. Worse, as Sena goes to cast the earthquake, a lone mage on the wall counterspelled the first one. If she persisted, she'd manage to get the next two off, despite the attempts to counter them, as well.