While the towering earthen walls of the Dimmock Line were a major factor in the Union army's failed attempts to capture Petersburg directly, the Federals wasted little time digging an opposing trench line. More than a hundred miles of trenches were dug, including the main and secondary lines.
Federal engineers tested a wide variety of fort designs during the siege. In total, they designed and constructed forty-one forts along the inner and outer Union trench lines, with additional forts built to protect City Point. Some of the more famous forts were Fort Stedman near the city, Fort Wadsworth on the Weldon Railroad, and Fort Fisher, the largest earthen fortification on the Petersburg front. Union engineers also built bridges and roads for the army, and sometimes digging mines.