Third System forts were designed to hold out for 14 days, and they did - all except two that had extenuating circumstances. This is despite the fact that the forts had smaller guns than the besieging forces. The Third System comprised a system of communication (roads, primarily) that would allow a relieving force to break the siege within the two weeks that the fort was designed to hold out. The Confederacy had no such relieving force, and the forts ultimately fell - as one would expect. It was well known at that time that a fort could not hold out forever, but these forts did their job.I've always been intrigued at the fact the United States spent a great deal of time and money building up third system fortifications prior to the war but when it came down to it, they really didn't stand up to the artillery of the time. Time and time again they were battered into submission (Fort Pulaski, Fort Morgan, Fort Jackson, etc) or virtually destroyed (Fort McRee, Fort Sumter) in short periods of time.
Meanwhile, the earthwork fortifications that ringed Charleston Harbor held out nearly the entire war.
Fort Sumter not only held out for 23 months, it was still a viable defense. When the Federal forces attacked with 200 marines, the marines suffered 200 casualties attempting to capture the "rubble fort." All were killed or captured. The design of the fort, with the casemates separate from the scarp, like all Third System forts, allowed the outside wall to fall while the casemates remained intact.
I've often said about the battles between the forts and the besiegers, that if you take a pistol and give me a sniper's rifle, I'm going to win the battle no matter how good a shot you are. That was analogous to what happened during the sieges of these forts.
As to the comments about the Charleston defenses being superior to the other Third System forts, that's simply not true. First, the same engineers designed these forts and laid out the defensive strategies. Second, the fact that Fort Sumter had an intrepid garrison was the major factor there - forts receiving significantly less damage surrendered where Sumter held firm. Of the two forts that didn't hold out for 14 days, Fort Gaines surrendered with very little damage and the fort commander was court-martialed by the Confederacy. Fort Jackson was built below the level of the Mississippi River, and Porter targeted the dike and flooded the fort. The defenders were walking around in sewage - overflow from the flooded latrines - and mutinied, demanding the surrender of the fort. New Orleans had already fallen, and the fort commander complied.
Forts of the Third System became obsolete in the 1880s with the development of massive breech-loading steel artillery. No system of defense can hold out forever; obsolescence is assured. That's why the original name of the Third System, the Permanent System, was a misnomer.