A sally port is protected point of entry into a secure location, such as a prison or a military fortification. Often, a sally port consists of an enclosed area with a solitary gate on either side, only one of which can be opened at any given time. A sally port might also be constructed in such a way as to block the line of sight from enemy fire. Many of the most secure facilities in the world, such as nuclear weapon storage areas, make use of sally ports to control the traffic of personnel and vehicles into and out of the facility.
Historically, a sally port was a guarded, often hidden gate in a castle or other defensive fortification from which a sally, or sortie, could be made. A sally is a type of surprise attack where troops issue forth from a defensive position, and it is often made by a besieged defender upon vulnerable attackers or valuable siege equipment. Such attacks allowed those in the besieged position to weaken the forces assembled against them without the greater risk of opening the main gates, and they were sometimes sufficient to break sieges altogether.
In modern times, sally ports are most commonly seen at high-security prisons, military bases or even courthouses, and they serve a number of purposes. In a courthouse, for instance, a prisoner might enter the court through a sally port directly from his or her transport vehicle, with entry controlled by the bailiff. This allows a prisoner to be transported to the interior of the courtroom without exposure to the public.
At most military sally ports, a guard is in charge of monitoring and controlling all traffic, commonly from a physically removed location. He or she retains the means to examine the credentials of any personnel and the contents of any cargo that might be passing through to the other side. The guard also is in charge of keeping traffic running smoothly.
Modern sally ports are often not suitable to a high traffic location, because traffic can flow in only one direction at a time. Additionally, great care must be given to sally ports in an emergency. A sudden onset in traffic can create a situation in which the sally port is overflowing, rendering the doors unable to function properly on either side, which causes the flow of traffic to stop altogether.