The Oxford English Dictionary (2nd edition) defines “postern” as a back door; a private door; any door or gate distinct from the main entrance; a side way. The OED gives as an example: “The gate is in 3 divisions, a carriage way and 2 posterns for foot-passengers divided by stone piers,” Elmes, Metropolitan Improvements, 1828. The OED give as a secondary meaning of postern a way of escape or of refuge; an obscure passage. The OED also says that the word served for literary use as a discrete euphemism for anatomical hind parts, as in: “Then in the Posterne of them looke, and and thou shalt find the Postume Poems of the Authors’ Father”, Ben Jonson, Crudities, 1611.
The City Ward of Cripplegate was divided by the City Wall, originally built by the Romans. The part of the Ward inside the wall was Cripplegate Within, and the part which had become inhabited in the middle ages outside the City wall, was called Cripplegate Without. The distinction remains to this day. The area commonly known as the Barbican more or less covers the same area as Cripplegate Without. The dividing line roughly ran along the route of the present London Wall. Click Stow’s description for a description from Elizabethan times which includes the Postern.
By the mid-17th century, the area immediately outside the City wall had grown so much that the two gates which existed, Cripplegate and Moorgate, were not enough to cope with the passage of people and vehicles between the two parts of the Ward. Two breaches were made in the wall and “posterns” or entrances created. It seems they were still not enough, because in 1654 a motion was put to the Court of Common Council, the City governing body, to insert a third postern between the two already built.
This was approved and a postern was inserted in the City wall at the northern end of Aldermanbury, roughly where the extension of the bridge over the lake in the Barbican meets London Wall. This was the Aldermanbury Postern and it is the particular postern from which The Postern in the Barbican takes its name.
These posterns proved their worth in 1666 when the population of Cripplegate Within were able to flee the Great Fire and escape into the Outer Ward. The City wall must have formed a fire break of sorts because the Outer Ward escaped the fire.
But the posterns were only narrow passages and by the mid-18th century they were inadequate, On 28th April 1752, a petition was presented by the inhabitants “near Aldermanbury Postern for the removal thereof, it being a nuisance.” It seems to have been removed in about 1753. An author named Entick, writing in 1766 said: “The narrow passages, which were at the posterns facing Aldermanbury and Basinghall Street are removed, and the passages made wide into Fore Street – besides another opening into the said street from Coleman Street through London Wall”.
Another opening was made in the Wall in 1877 through St Giles’ churchyard. Much of what was left was destroyed in the bombing during the War.