This is a beautiful linen type postcard Fort Massachusetts on Ship Island, which is off the Mississippi coast. The card was never used.
According to Mashburn's
The Postcard Price Guide, A Comprehensive Reference,
the Linen Era postcards were produced between 1930-1945. The paper has a linen-like texture, which gives this type of card a distinctive appearance.
Construction began on the Ft. Massachusetts in 1859, and wasn't complete when the Civil War started. It was eventually abandoned by the Confederate forces, and the Union forces later held it, launching their invasion of New Orleans from Ship Island.
Ship Island and Ft. Massachusetts are now part of the Gulf Islands National Seashore.
Andy Bennett has some great photos of Fort Massachusetts on his website, Coastal Fortifications on the Gulf of Mexico.
You can visit Ship Island with Ship Island Excursions.