The eastern portion of the Castillo's moat was filled in by the US Army from 1842 to 1844 in order to serve as a water battery. Just about every fort built by the United States in their Second and Third Systems of Seacoast Defense was issued one of these things, a Hot Shot Furnace!

Solid cannonballs would be rolled into this structure where they would be heated to an attractive shade of cherry redness, whereupon they would be extremely carefully loaded and fired at an attacker's wooden ships. This flaming ball would then, notionally, wreak the kind of firey havoc that one imagines would be found distasteful by any ship's crew.