The granary primarily served to store supplies in a strongly built structure with a stone floor, double rows of adobe brick walls, and an exterior adobe floor that helped keep water out. Its careful construction protected food and other necessities stored inside and minimized rodent activity. As one of the strongest buildings constructed, it also served a defensive purpose, with a roof and added parapets that allowed it to function as a bastion protecting San Lorenzo's northern and eastern sides. The parapets provided cover for small cannons used atop the granary during attacks. Next to the granary, a passageway led to a small gate known as a postern that allowed access to a spring and was easily defended. To the south were several structures that could also hold supplies, including one with a strong stone foundation that was likely used as a powder magazine for storing ammunition and gunpowder.