Fort Knox's roof, or terreplein, had emplacements for thirty cannons, smaller than the guns planned for the casemates on the main level below. All thirty gun circles on the terreplein were laid and ready for guns in September 1864, but the fort was not fully armed and these emplacements never received cannon. Other areas, including the batteries and the main level's casemates, were given higher priority for defense and received most of the seventy-four cannons eventually mounted at the fort. Terrepleins at forts like Fort Knox were usually covered with earth because it was inexpensive, easy to level, could absorb the impact of shells from enemy ships, and was not easily damaged by vibrations from the fort's own cannon fire. In 1996, a major project began to waterproof the terreplein, repair damaging leaks below, and improve drainage by creating a new concrete surface and drainage beneath the earthen fill to bypass the deteriorated drainage system deep inside the fort. Machinery removed the fill down to the top of the casemate arches on the main level below. These arches had originally been coated with concrete and mastic to prevent leaks through the brick masonry, and engineers and workers in the 1860s had installed vertical drains between each casemate to collect water from the terreplein and direct it outside the fort or to cisterns inside. When first built, the drains were capped with removable circular covers for inspection. After the fill was removed in 1996, a concrete slab was poured, a waterproof membrane applied, and a drainage mat and drain pipes installed before the earthen fill was replaced and grass planted.