And what a hot shot furnace it is. You're surely wondering what that sign says:

U.S. SHOT FURNACE

This structure is a hot shot furnace for heating cannon balls to
be shot at wooden vessels and to set them on fire. It is part of the
water battery built by the U.S. (1842-1844) when this side of the
moat was filled and guns were mounted on the stone arcs behind
the sea wall.