There were no armaments until Anderson moved in.
it was an empty shell of a fort. And most of the emplacements for cannons and artillery faced out towards the Atlantic. There were few spots to put large guns facing towards Charleston.
So, it was the South Carolinian militia that shot Anderson's men in the back. Specifically, at 4:30 a.m. on April 12, 1861.
Anderson was able to bring a total of 60 guns to Sumter. Only 21 of those were able to be positioned to fire into the bay. From memory, he had 200 men. I can be wrong about that though.
By comparison, Beauregard, leader of the South Carolina militia, built-up overwhelming strength to challenge Fort Sumter. He had three 8-inch Columbiads, two 8-inch howitzers, five 32-pound smoothbores, and four 24-pounders. He seized the US Army Fort Moultrie (the one that Anderson had abandoned).
Outside of Moultrie were five 10-inch mortars, two 32-pounders, two 24-pounders, and a 9-inch Dahlgren smoothbore. The floating battery next to Fort Moultrie had two 42-pounders and two 32-pounders on a raft protected by iron shielding.
He also seized Fort Johnson, another US Army fort. Fort Johnson on James Island had one 24-pounder and four 10-inch mortars.
At Cummings Point on Morris Island, Beauregard had emplaced seven 10-inch mortars, two 42-pounders, an English Blakely rifled cannon, and three 8-inch Columbiads, the latter in the so-called Iron Battery, protected by a wooden shield faced with iron bars.
He also had about 6,000 men were available to man the artillery and to assault the fort, if necessary, including the local militia, young boys and older men.
So much for that "oh, those poor Southron boys, just tryin' to defend their homes from those Northern aggressors!"
They brought in Roger Pryor, a noted Virginia secessionist and super supporter of slavery, to fire the first shot. You don't get more symbolic than having a rabid supporter of slavery firing the first shot at US troops than that.
it was 100% about owning slaves. The southern economy was built on slavery. Their treasonous statements of secession specifically listed slavery as a primary motive.
it was about owning people, their children, and their children's children and profiting from their forced labor.