Sallyport , noun;
1. An opening in a fortified place from which troops may make a sally
This word is first attested in 1651 and is a combination of sally + port. A sally is "an issuing forth," which was first attested in 1560 from French saillie ("issuing forth"), which derives from Old French sailir ("to dance"). Port is "a gate, gateway" and goes back to Old English from classical Latin porta ("gate, opening, outlet").
Nowadays, a sallyport is a secure, controlled entryway, particularly at a prison or fortification.