To find such a wide break in a fort's wall seems strange to a person today. Yet the gap you see here - then called a sortie passage - is a carefully calculated part of the defenses of Fortress Rosecrans.
The area close to the walls was covered by cannon fire from many nearby angles. Enemy troops could not reach this point without suffering massive losses. With all the nearby trees and brush cleared away, Fortress Rosecrans had no blind spots. No enemy came close.
The Lunette [if] attacked repulses the enemy by its direct fire of musketry and artillery; the collateral Lunettes and curtains cross their fire in front of it [if] the enemy attack one of the curtains... the enemy is met by direct fire ... and cross fire from the ... Lunettes.
James St. Clair Morton, brigadier general
Chief Engineer, Army of the Cumberland
Civil War cannoneers could fire solid shot, exploding shells, or case shot at long ranges. When the enemy approached within 400 yards, the gunners switched to canister or grapeshot, sending a deadly shower of iron balls into their ranks.
The sortie passages ... are designed to admit, or give exit to, large masses of our own troops, in the case of a general engagement taking place...