bailey

Bailey is a protective wall surrounding a castle or the outdoor space created by the castle's protective walls. (noun)

In a movie where people are fighting from the top of a castle wall trying to keep out the approaching enemy, the outermost wall is an example of a bailey.
An example of a bailey is a courtyard inside the protection of the castle walls where the ladies of the community gather daily to converse.

See bailey in Webster's New World College Dictionary