Writer’s Deep Dive: Posterns
Castles and fortified cities are popular with writers. Access to them can become critical to the plot, especially during a war or siege. Gates and barbicans are well-known access points, but many castles and cities use posterns as a secondary point of entrance or exit. Let’s dive in!
Posterns, also known as sally ports, were secondary doors or gates. They were set into the outer curtain wall of a castle or city. They were usually small, with only enough room for one person to pass through at a time, although some were large enough for a horse and rider. Often, they were concealed, making it difficult or impossible for someone unfamiliar with their location to find them. They commonly had gates or doors that locked or latched shut.
Because of their size, they were easier to defend, since only one soldier or rider could come through at a time. This created a chokepoint that favored the defenders. 
Posterns had a varied of uses, including as a sally port during a siege to attack the besieging army, a discrete way of entering or exiting, and an escape route.
There are historical records of posterns and several that still exist. The cities of Jerusalem, London, and York had many posterns. Posterns are also mentioned in literature. In Le Chanson de Girart de Roussillon, the hero escapes through a postern when he is betrayed. In Sir Thomas Malory’s Le Morte d’Arthur, a knight of the Round Table flees through a postern. 
The Write Angle
There are so many opportunities for a writer when it comes to posterns.
If you have a character that needs to escape from a castle or walled city, a postern provides a great option. Since some of them were large enough for a horse and rider, messengers could use them to escape a besieging army, especially since posterns were well away from the main gates and commonly concealed. In fact, in my in-process novel, I use a postern as a means of escape for several messengers fleeing a besieged city.
A postern could also be used to get into a castle or walled city. If a person is told of the location of a concealed postern, they could sneak in. The door could be opened from the inside or the lock could be picked from outside. It could also be used by a resident of a castle or walled city to sneak out to meet a sweetheart or deliver covert information.
Of course, as I mentioned above, posterns were used by soldiers to sneak out and launch surprise attacks on an enemy army, especially if they are besieging the castle or city. A castle’s soldiers could also sneak into an enemy’s encampment and sabotage their tools and equipment, turn their horses or other livestock loose, or set things on fire.
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Copyright © 2022 Rebecca Shedd. All rights reserved.
 Van Emden, Wolgang. "Castle in Medieval French Literature", The Medieval Castle: Romance and Reality (Kathryn L. Reyerson, Faye Powe, eds.) U of Minnesota Press, 1991, p.17 ISBN 9780816620036  Malory, Thomas. Le Morte D'Arthur, Chap IV, Library of Alexandria, 1904