The primary modern meaning for sally port is a secure, controlled entryway, as at a fortification or a prison. The entrance is usually protected in some way, such as with a fixed wall blocking the door which must be circumvented before entering, but which prevents direct enemy fire from a distance. It may include the use of two doors such as with an airlock. Many times they are driven through secured garages designed to pick up and drop off arrestees.
For more information visit Wikipedia.