sally port

noun2

1. •
   An opening into or entry to a fortification, usually arched, to enable a sally; a postern.

   • And so usually a trusted caporegime would be sent out to rent a secret apartment and fill it with mattresses. That apartment would be used as a sally port into the city when an offensive was mounted. — 1969
2. •
   An entryway controlled by two doors or gates, each of which must be closed before the other can open.