Final answer:
Darius I used his army and military garrisons as the main tool to keep his empire together and maintain control over the satraps. Audits and reports conducted by royal officials helped ensure governance compliance.
Step-by-step explanation:
The main tool used by **Darius I** to keep his empire together was his **army**. Darius I, the king of Persia, employed a system of military garrisons to enforce laws, maintain order, and discipline satraps, who were the regional governors. These military garrisons answered directly to the king and could remove or execute satraps if their governance was deemed unsatisfactory.
Furthermore, Darius I sent royal officials to conduct audits and compile detailed reports about the governance of satraps. These reports were sent to the king for review, and negative reports could result in removal or execution of satraps. By maintaining a strong army and deploying effective strategies to ensure loyalty and obedience from the satraps, Darius I was able to keep his empire intact.