Directly behind the southern bastion is this detatched magazine, the smaller of two built within Fort Jefferson. A starfort's magazine was generally built into the bowels of one or more of its bastions, as those were the most stoutly-constructed parts of a starfort...but the smaller bastions of the polygonal fort were more about protecting the fort's flanks than projecting firepower outward (polygonal forts were also known as flankless forts), so they wouldn't have withstood as much damage as a classic starfort bastion. Why stick all your gunpowder in a relatively dinky bastion, then? Fort Jefferson's designers did, in fact, also build 32 smaller magazines within the fort's walls, which the larger detatched magazines kept supplied.