Early in the Revolutionary War, the Continental Congress realized that the Hudson River was critical to the American Cause. If the British controlled the river, they could divide the rebellious colonies. Therefore, the Americans began work on Fort Montgomery in March 1776 and quickly erected several buildings. Sited where the river is narrow and currents made navigation difficult, the fort was originally conceived as a large battery of cannons. At its heart was a "Grand Battery" of six 32-pounders commanding a long stretch of the river. As work progressed, however, it became clear that the fort would be attacked from the land. Therefore, the earthworks were slowly extended to enclose the fort.
When the Americans discovered that the land on the opposite side of the Popolopen Creek was higher and would threaten Fort Montgomery if held by the enemy, they began constructing a second fort there, called Fort Clinton. They connected the two forts by a pontoon bridge. As labor was concentrated on the new fort, work on Fort Montgomery slowed. Other problems affected construction of the fort too. Supplies were often hard to obtain, morale was often low, and discipline was a chronic problem. Nevertheless, Forts Montgomery and Clinton were largely complete by October 1777, when the British attacked them.