Fort Sumter is the most heavily shelled structure in the U.S. As a result of the repeated attempts to break down these walls and the Confederate spirit, the fort withstood much damage. So much so that, once abandoned, the fort was nearly in ruins.
Much of the fort has been rebuilt and looks almost nothing like the original version. But some of the walls that remained standing, albeit much shorter than they had been, remain as part of this now historic site. While touring the fort, you can see the indentations from canon fire. In some areas, canon fire smashed completely through the brick. Confederate troops and, perhaps, ironically, slaves, worked to plug these holes using sandbags, in an attempt to hold down the fort.
Fort Sumter was not heavily armed at the start of the civil war. In 1860, only 15 canons were ready for battle. After the Confederates seized the fort, they began fortifying its defenses. But some of the canons you see during the tour are Union canons which were located on nearby Morris Island and used to fire on the fort.
There is an impressive assortment of canons scattered throughout the ground and top levels. Most contain a description of their range of fire and how they were used.