Before airplanes and missiles, the United States defended itself with fortifications along its coast and harbors.
In this harbor a system of forts stood ready to protect New York; nearly a dozen were built before the War of 1812. Cannons in the forts combined to create a deadly, interlocking field of fire that could stop invading warships.
"The City and Harbor of New York have been lately fortified against the assaults of a naval enemy at a very great expense. [The forts] mount about 300 cannon and mortars, for the working of which 3,500 men are required."
- A Gazetteer of the State of New York, 1813
The guns of Castle Williams, together with the other forts, created overlapping fields of fire with five other forts (Jay, Wood, Gibson, Clinton, and South Battery).