When one is interested in firing a cannon from a casemate, one needs to somehow deal with the copious gunpowder smoke that is produced with each discharge. The attempted solution generally took the form of some sort of opening above the firing embrasure, which was designed to vent the smoke out of the casemate. As far as I can tell, it was never really possible to effectively vent that smoke out of a casemate, which would have led to the rapid asphyxiation of many a gun crew, had guns ever been fired from casemates in the originally intended fashion, which for the most part they were not. Very few American starforts ever saw action of any kind, but those that did fired their guns en barbette, from atop their walls and bastions.

However! When these starforts were constructed, the expectation was that they would be firing their guns from inside the casemates, so vents were duly built into each. The style of these vents varied from fort to fort, but the vent represented here, which was repeated above each and every firing embrasure, is peculiar to Forts Macomb and Pike: Again, as some of the first forts built under the Third System of American Seacoast Defense, there are some weird contrivances at work at these two starforts.