[Non-Sequitur]

It was more indefensible than Moultrie. Since I lived their for several years I often looked out across the marsh towards Mount Pleasant S.C. and wondered to myself "why leave here to go there"?

Things were apparently different then. Several books on the Sumter crisis describe conditions as Moultrie as indefensible. Civilian properties had grown on both sides of the fort, and wind had pushed sand dunes up to the top of the walls. There had apparently been cases where cows grazed up the sand dunes and wound up in the fort's parapet.

[TexConfederate1861]

Moultrie is STILL that way today.
One look will tell you why Anderson chose to move.

[smug]

Several books on the Sumter crisis describe conditions as Moultrie as indefensible.

The fort itself is as you described (minus the cows). If as some of the reports had it that no southern troops were on Sullivan's, it is my contention that the whole island could have been defended (longer). Federal ships could have used the island as a screen to reinforce and assist. The southern patriot's would then have been more limited in their attack.