For permanent defensive positions, both the Union and Confederacy designed fortifications with a wide variety of structures and obstacles. After the ground was cleared by hand of natural and man-made obstructions in front of more permanent field works, additional obstacles could be used to impede attacking troops. The wide, flat inside floor at the fort's center was the parade ground. Other features included a bombproof, a protective wood enclosure covered with earth that sheltered artillery crews during enemy shellfire; an embrasure, a break in a wall that allowed a weapon to fire while protecting the gun crew; chevaux de frise, a portable anti-cavalry frame covered with projecting iron or wooden spikes; and a traverse, a mound of earth used to protect gun positions from explosion or shield the inside of a fortification.