sally port

a gateway permitting the passage of a large number of troops at a time.

a postern.

Origin of sally port

First recorded in 1640–50

In the outwork was a sallyport corresponding to the postern of the castle, and the whole was surrounded by a strong palisade.

Ivanhoe |Walter Scott
It was a species of free-masonry known only to the initiated at the Sallyport.

Snarleyyow |Captain Frederick Marryat
A heavy division forced its way into the redoubt through the sallyport, and as Ned saw he uttered a deep gasp.

The Texan Scouts |Joseph A. Altsheler
He turned around to laugh significantly, then walked away towards the sallyport, swinging his hanger contentedly.

Cardigan |Robert W. Chambers

Ten minutes later, the "assembly" sounded to form ranks on the parade outside of the sallyport.

Ten years in the ranks, U.S. army |Augustus Meyers

sallyport

/ (ˈsælɪˌpɔːt) /

an opening in a fortified place from which troops may make a sally