There's not a whole lot left of what was the Castillo's, or more accurately Fort Marion's, water battery. Fort Marion was of course the name the United States gave the awesome new Spanish starfort they acquired in 1821, since we couldn't have an active American fort with a Spanish name, now could we.

The fort was named for US Revolutionary War hero Francis "The Swamp Fox" Marion, who gave the British loads of trouble in South Carolina during that conflict. While Marion sounds like a cool dude and all, Fort Swamp Fox would've been a much better name, and it's probably best all the way around that the fort officially reverted to its real name in 1942.