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Originally a term from medieval fortifications, a sally port is a controlled penetration in a security barrier. In castles, it was a space between two extremely stout doors, comparable to the main gate, between which soldiers could "sally forth" to counterattack besiegers.
The term has come into use for penetrations of walls or other barriers in high-security prisons. Think of an "air lock"; both doors cannot be open at the same time. One side is considered secure, and, should unauthorized persons enter through the far door, defenders can block them with the other door. The guards usually have tear gas dispensers, firing ports, or other increasingly potent deterrents to anyone in the area between the doors.