Eustathius is contrasting a military general who captures a city once with the person who must continually rule the passions throughout life.
A general captures a city once and then orders it as he pleases. But the charioteer of the passions must keep bringing them into subjection for as long as he lives. The internal enemy is altogether harder to face than the enemy outside. The man who has sacked a city is free from care afterward, but the one who faces an unceasing enemy must keep struggling. To master anger, then, is a great achievement, and a great victory over the spirit of anger.