"determined to see their brother and Mr. Campbell to the salley-port;"

Plaque commemorating the Sally Port at Portsmouth
Creative Commons Attribution Share AlikePlaque commemorating the Sally Port at Portsmouth - Credit: Secret Pilgrim
 In general a sally port is an opening, such as a gateway or passage, which allows troops/sailors to leave a fortified location.  In Portsmouth the Sally Port was an opening in the garrison walls just adjacent to the Square Tower.

Click here to see a picture of the Sally Port in 1960.