The critical fortification for the American forces during the siege at Yorktown was Redoubt 9.

This earthwork was crucial to the American strategy, as its capture provided the Allies with a major advantage in artillery placement. The successful attack on Redoubt 9 took place on the night of October 14, 1781, and was instrumental in the broader siege efforts. By seizing this fortification, American and French forces could more effectively bombard British positions, ultimately leading to General Cornwallis's surrender and reinforcing the momentum toward American independence.