The sturdy structure in the southeast corner of the fort was used to store gun powder and ammunition. Because these materials were very dangerous, the building had double walls and a vaulted stone roof designed to contain an explosion in case of an accident. The double walls also had air vents to keep the interior from overheating. The vents on the inside walls did not line up with the vents on the outside walls, preventing rain and stray sparks from being drawn into the building and damaging the powder or causing an explosion.