From 1859 to 1876, army engineers built 30-foot-high granite walls to form this corridor. In military terms, Fort Tompkins’ scarp, the wall to the right, faced a counterscarp, the wall to the left, creating a 30-foot-wide dry moat. When originally built, the counterscarp extended behind the fort to protect its back and sides from land-based attacks. Within the counterscarp, a gallery allowed soldiers to shoot through the wall’s vertical slots as well as from behind the fort’s scarp wall through alternating gun slots. Attackers crossing the moat would face a deadly crossfire. Fort Tompkins and its defenses were never challenged, evidence of their effectiveness.
