The powder magazine was recommended by Harry Gordon and was built under the command of Captain Trump on May 6, 1758.  It is the only remaining portion of the fort. From the outside it appears as a small mound of earth and is descended into by a stone stairway. The stone construction is 10x12 feet and has a brick ceiling. At one time a wooden building stood over the underground chamber and for a short time it was enlarged and strengthened so as to house the first jail in Northumberland County.