The defense from artillery fire is counter battery artillery fire to knock out those artillery pieces. Now if you are using a cannon which can be aimed directly at your target that is one thing, but How do you "aim" a 10 inch columbiad gun buried halfway into the ground as a mortar. The only way you can control that shot is by adjusting the amount of gunpowder that you load it with. That wasn't a precise science in that day and time especially with a makeshift arrangemment as Anderson was describing. Therefore the accuracy of those shots would have been very suspect as to their intended target.
As David U. points out that these 10 inch guns may have been the only weapons that could have reached East Battery in Charleston. But could the East battery guns have reached Fort Sumter, if Sumters other weapons could not have reached the East Battery? So following the "counterbattery defensive" goes by the wayside if Charleston's east battery guns were no threat to Sumter. Why would you waste you ammunition and time shooting at weapons whose shells would fall into the water harmlessly short of their target?
Fort Sumter's baseball plate shape was designed that way because the intended defense of Charleston Harbor was supposed to be a defense in depth. That if ships were able to penetrate the harbor past Forts Moultrie and Sumter that they would be met by another line of batteries such as Castle Pinckney and the east battery, that even reach up the Ashley and Cooper Rivers. All the time while the guns of Fort Sumter could still fire upon the ships rear.
It is obvious that Major Anderson was using every thing in his reach to prepare to resist any attempts to reduce Fort Sumter. That can be considered as being defensive in nature. But to the people who those "defensive" guns are pointed at might have a different opinion. At the same time Anderson was doing his improvements the Confederate authorities were trying to negociate a peaceful solution to the problem with Abe Lincoln to no avail.
The fact that these Peace commissions were not being recieved by Lincoln could indicate only one thing. That Anderson "defensive improvements" were actual intended to be "aggressive" in nature. Were the Confederates placing new batteries? Yes they were. For every action there is an equal and opposite reaction. That postulate works as well in human relations as it does in physics.
No in my opinion this idea that Anderson was making all these improvements as defensive measure is a bunch of sliced salamy to me. He fully intended to aid in any way he could, by inflecting as much damage as he could, any attempt to resupply and reenforce his garrison at Fort Sumter with the intentions of holding that fort and chocking of the maritime trade to Charleston.