Report from the Fort21 January 1802 · Major J. J. Ulrich Rivardi
[of the fort walls] ? has been added on the north and west side ? These new works extend along the bank 1014 feet. They have a sally port on the west with a small bombproof for guards on each side of the gate, and a similar one in the center of the curtain - that on the west wants to be completed, 30 cubic feet of brickwork and thirty and a quarter feet of stone topping 4 by 12.
The verb SALLY in the English language means to rush forth or leap out. In old English "sally forth" meant to go hurriedly with energy and movement. Military usage of the word sally meant to rush out suddenly to attack besiegers. In fortification terminology, a sallyport was the name for the tunnel of a fortification through which such a "sally" was made. The opening of the tunnel through the fortification wall was protected by heavy metal-studded doors with massive hardware for security. A drawbridge and moat would impede attackers before they reached the fortification. Guardrooms inside the walls held armed soldiers. During wartime they would challenge suspected aggressors. In peacetime they monitored the movement of the garrison (deserters were frequent) or local hucksters trying to enter to sell their wares.
As described above by Rivardi, Fort Mifflin has two sallyports, this one and the one in the north wall. Each has small guardrooms or casemates with fireplaces off the sides. Gun portals or embrasures ensured opportunities for protection. Sallyports in masonry walls of a fort served as critical openings for defense tactics. Garrisoned troops inside the fort could burst forth from a guarded portal to attack the enemy. The defenders and the enemy used the various angles of the fort walls for protection, but the defenders could achieve a pincher movement against unsuspecting enemy using the sallyports. The garrison could return through a sallyport to rearm and then siege from a different sallyport. This advantage could force the enemy into a locked position and achieve victory for the defenders. However, because no war was ever fought around the fully enclosed fortification walls and sallyports that exist today at Fort Mifflin, its 18th-century military defense plan has never been tested.