The first shots of the Civil War rang out at Fort Sumter, on a pentagon-shaped island in the harbor. A Confederate stronghold, this fort was shelled to bits by Union forces from 1863 to 1865. A few original guns and fortifications give a feel for the momentous history here.
The only way to get here is by boat tour, which departs from 340 Concord St and from Patriot's Point in Mt Pleasant at varying times depending on the season (check the website). The monument also includes Fort Moultrie on Sullivan's Island.