sally port

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gate in a fortification designed for sorties.

from The Century Dictionary.

• noun In fortification, a gate or a passage to afford free egress to troops in making a sally.
• noun A large port on each quarter of a fire-ship, for the escape of the crew into boats when the train is fired.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small door in a fort or a castle to enable a sally; a postern.
• noun An entryway controlled by two doors or gates, of which each must be closed before the other can open.