Think of people defending a strong city. They mock the enemy soldiers trying to attack them: "Look at those fools wasting their time. Our walls are solid. We have plenty of food. We have everything we need." That's what makes a city impossible to conquer. For you, it's your opinions that make your soul impossible to conquer. What wall is stronger? What body is tougher? What possession is safer? What reputation is more secure than your own thoughts?
In this way also those who occupy a strong city mock the besiegers (and say): What trouble these men are now taking for nothing; our wall is secure, we have food for a very long time, and all other resources. These are the things which make a city strong and impregnable; but nothing else than his opinions makes a man's soul impregnable. For what wall is so strong, or what body is so hard, or what possession is so safe, or what honor (rank, character) so free from assault (as a man's opinions)?