The nation's Third System forts, including Fort Knox, all shared similar design features. These features combined to guard against attack by ships, make it impossible for the fort to be taken quickly, [and] provide protection from a land-based, long-term siege.
To fulfill these purposes, forts like Fort Knox provided a variety of places where cannons could fire and be protected. Fort Knox was planned for 135 cannons. With these cannons in the main structure and outworks, or batteries, Fort Knox's design used principles first developed by the 17th century French engineer Marquis de Vauban. He believed that fortifications should consist of multiple layers of defenses that attackers would have to conquer one at a time. Designers of Third System forts envisioned these layers of defense as a means of delaying attackers until outside reinforcements arrived or the attackers' strength ran out.
The Civil War brought many changes that made large masonry forts like Fort Knox obsolete. New armored and faster steam-powered warships were more elusive targets for fort cannons. The armaments on these vessels had also improved. Rifled cannons, with spiral grooves in their bores, could fire newly-developed projectiles with more force and accuracy than older smoothbore cannons. As a result, shots from rifled cannons could quickly reduce a fort's masonry walls to rubble.
Engineers unsuccessfully experimented with applying iron armor to walls of masonry forts so that the walls could better repel the powerful projectiles fired by ships' rifled cannons. At the same time, new rifled cannons were developed for forts. These new land-based weapons were larger than the old smoothbore cannons and required new emplacements. The new cannons meant that new forts were designed, built, and sited differently than the old Third System fortifications. Never again would large, beautifully-built masonry forts like Fort Knox be part of the nation's modern coastal defenses.
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