An 1865 photo of the ruins of Roman Catholic Cathedral of St. John and St. Finbar destroyed during the Great Fire of 1861. You can see more photos of the fire's aftermath here.
Long before Charleston fell to the Union, the Great Fire of 1861 did nearly almost as much damage as the next four years of siege would.
It started late on the night of December 11, 1861, and would cut a swath through the heart of the city leaving the city more dazed than an encroaching Union force from Beaufort had yet managed.
By the time it was out a third of the city had burned.
The Post and Courier has dug up an account of the tragedy and related unfortunate losses during the surrounding time period; take a read here. And below is a map of the fire's stretch with some markers to help you understand how the fire moved.
And if you'd prefer an alternate account, check out this section in "The Siege of Charleston."