Fort Mifflin's West Sallyport. This was added around 1798 when the fort's walls were being completed. The concept of the sallyport was to serve as a protected position from which the defenders could burst forth, unexpected, to attack a conveniently surprised and unprepared beseiging enemy. It would seem to me that such an idea made rational sense before the advent of firearms, but if you had only three points from which defenders could conceivably "sally forth" and attack, a few of the enemy's guys with guns covering each of these portals would make short work of any such "surprise." Unfortunately, the swordsmen of William the Conqueror (1028-1087) never attacked Fort Mifflin, otherwise this plan would have no doubt been successfully carried out.