Another vestige of the original intent to fire heavy guns from inside casemates is this, the vent hole. Every American starfort that was designed to pour cannonfire from casemates had to make allowances for the fact that black powder smoke is murderously dangerous to those who must breathe a lot of it, and a variety of vent holes such as that above were thought to be the answer. There are almost as many different styles of vent hole as there are different starforts, but this diamondesque arrangement is fairly common.