A bailey is the sturdy wall around a castle that keeps invaders out. The bailey of a medieval castle was usually built of stone.
You might see a bailey — or the remains of one — if you tour a castle in England or France. The word bailey is used both to describe the defensive wall around a castle courtyard, or sometimes the courtyard itself. This type of castle design was especially common in twelfth century England. The word comes from the Old French bail, "stake, palisade, or brace," which may be rooted in the Latin bacula, "sticks."