The Engine was a zenmensonsou, a sentient siege engine located in the west tower of the Wall of Bones. It was a massive weapon of blackened and melted steel that seemed to grow out of the piled bones that made up the tower. The Engine was a weapon of ranged destruction, which pivoted itself to slam into any hostile forces that get near it. It required the full concentration of one spellcaster capable of casting powerful maho spells to function. The engine was the creation of Kaiu Yohiro, a Lost engineer follower of Daigotsu.