From a US Marine Corps seminar: "Defense of a locality by troops against the assaults of their opponent render the locality subject to bombardments. Factories producing munitions and military supplies, ports and railroads being used for the transportation of military supplies, and other places devoted to the support of military operations or the accomodation of troops may be attacked and bombarded even though they are not defended."
I do believe the city of Charleston was being defended against assault. As such, it was a legitimate target, even using today's standards. I believe most of Charleston was attacked by Union guns at their extreme range, which at the time meant marginal accuracy. Were not Confederate batteries emplaced at many locations in the city and the entire city occupied and held as a fortress against assault?
BTW, do you have any compilation of the total number of dead civilians in Charleston during the siege?
The Feds' normal procedure was to target a church steeple in the civilian area of Charleston, not any port area. The Feds also shelled Fort Sumter for years and even launched an amphibious attach against the fort, but they couldn't take it. Other forts and Confederate troops outside of the city defended the harbor. Why not attack them rather than the civilians?
Perhaps you'd be interested in Union General Gillmore's demand regarding firing on civilians, which he commenced at 1:30 AM shortly after sending this communique.
HEADQUARTERS DEPARTMENT OF THE SOUTH, Morris Island, S. C., August 21, 1863.
General G. T. BEAUREGARD,
Commanding Confederate Forces, Charleston, S. C.:
GENERAL: I have the honor to demand of you the immediate evacuation of Morris Island and Fort Sumter by the Confederate forces. The present condition of Fort Sumter and the rapid and progressive destruction which it is undergoing from my batteries, seem to render its complete demolition within a few hours a matter of certainty. All my heaviest guns have not yet opened.
Should you refuse compliance with this demand, or should I receive no reply thereto within four hours after it is delivered into the hands of your subordinate at Fort Wagner for transmission, I shall open fire on the city of Charleston from batteries already established within easy and effective range of the heart of the city.
I am, general, very respectfully, your obedient servant,
Q. A. GILLMORE
In other words, give up your forts or I'll kill your civilians. How civilized.
I don't have any totals on Charleston civilian casualties. What I posted came from a couple of books I own and Xerox copies of a few old newspapers.