TravellingHat makes a very good point. I personally always make a point to bring workers on all my military campaigns. Then once my SoD is adjacent to the enemy city I intend to capture, I have my workers build a fort while my trebuchets reduce their walling to rubble. Doing so greatly increases the survivability of your troops during the time it takes for you to bombard the city's defense to 0%, especially if you're fighting a mad dog like Shaka who WILL throw wave after wave of his troops at your SoD like, well, a mad dog.