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

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

The principal entrance is on the south-east side, and directly opposite it is a sally-port.
A Yacht Voyage Round England|W.H.G. Kingston
We shall soon learn that the sally-port at La Bassée was the scene of long and desperate struggles.
The Childrens' Story of the War, Volume 3 (of 10)|James Edward Parrott
There was no sally-port in all this wall save one—the one that death had promised to open at the dawn.
The Master of Appleby|Francis Lynde
"Here is a sally-port ready provided for our sortie," the stranger coolly observed, motioning to the other to precede him.
The Wept of Wish-Ton-Wish|James Fenimore Cooper

Throw open the sally-port; to the field, Sixtieths, to the field; pull not a trigger, lest ye kill my lambs!
The Last of the Mohicans|James Fenimore Cooper