Shi Huangdi, the first Emperor of China, tore down the walls of Chinese kingdoms primarily because he wanted to ensure the nobles could not defend against him. By dismantling these walls, he aimed to unify China and prevent any regional powers from resisting his rule. This action was part of his broader strategy to consolidate power and create a centralized state. Therefore, the correct answer is:
- He wanted to ensure the nobles could not defend against him.