Two things you might note in this picture: The single opening for the hot shot furnace's firebox (some shot furnaces have two openings, the actual firebox and a repository for the resulting ashes beneath), and the red edges on the star braces.

When I first noted what looked like the remains of red paint on these star braces, I remembered that, in the Castillo's glory days of the Spanish Empire, the fort itself was plastered white, with its four garitas, the one-man sentry boxes on each of its pointy corners, painted red to symbolize the Spanish flag. Had this hot shot furnace been painted white, with its star braces painted red? Well of course not, because this shot furnace was of American construction, and those manning Fort Marion in the 1840's would have had no interest in glorifying the Spanish Empire via a colorful hot shot furnace.

The red that shows on some of the star braces' edges is in fact primer paint, visible only because the outer coat of black paint has worn away. Time for a fresh coat there, National Park Service!