According to a nearby information board:

Fort Montgomery’s powder magazine provided secure, dry place in which to store the garrison’s gunpowder and ammunition. The magazine was located here because of the site’s good drainage and because of the protection afforded by the rock ridge between it and the river. The magazine was build early in the fort’s construction. By the middle of June 1776, its 8-foot thick walls were completed, and masons were ready to finish its arched brick vault. Once finished, the arch was covered with sandy soil and a roof was built over top to finish the magazine.

Some of Fort Montgomery’s powder was produced a Henry Wisner’s powder mill 20 miles away in Goshen, New York. The loose powder was stored in the magazine in 100-pound barrels until it was used to make cartridges for muskets and cannons. This magazine served both Forts Montgomery and Clinton. During the battle, however, the British cut off communications between the two forts by capturing the pontoon bridge. When the British overran Fort Clinton, its garrison was running out of ammunition.