The nation's Third System forts, including Fort Knox, shared similar design features intended to guard against attack by ships, prevent a quick capture, and provide protection from a land-based long-term siege. To serve these purposes, forts like Fort Knox provided a variety of protected places from which cannons could fire. Fort Knox was planned for 135 cannons, and with these guns in the main structure and outworks, or batteries, its design used principles first developed by the 17th century French engineer Marquis de Vauban. He held that fortifications should consist of multiple layers of defenses that attackers would have to conquer one at a time, and designers of Third System forts saw these layers as a way to delay attackers until outside reinforcements arrived or the attackers' strength was exhausted. The Civil War brought changes that made large masonry forts like Fort Knox obsolete. New armored and faster steam-powered warships were harder targets for fort cannons, and their armaments had also improved. Rifled cannons, with spiral grooves in their bores, could fire newly developed projectiles with greater force and accuracy than older smoothbore cannons, and shots from them could quickly reduce a fort's masonry walls to rubble. Engineers unsuccessfully experimented with applying iron armor to masonry walls so they could better repel the powerful projectiles fired by ships' rifled cannons. At the same time, new rifled cannons for forts were larger than the old smoothbore weapons and required new emplacements. These changes led new forts to be designed, built, and sited differently than the old Third System fortifications, and large masonry forts like Fort Knox no longer formed part of the nation's modern coastal defenses.