From backside postcard:
The Fort was named after the Revolutionary War General Thomas Sumter, it was one of several defensive sites located along the eastern coast of the US. Construction began in 1829 and continued for over thirty years until it was finally completed in 1861. Fort Sumter was built on a sand bar located at the entrance to the harbor in the Charleston. Several thousand tons of granite were used to create a five-sided 170 feet by 190 feet long structure made with walls that were five feet thick, it was an impressive fortress and an effective first line of defense against attacking forces in the region.
"The Moon" Global Forever stamp