The space enclosed within the external walls of a castle, not
including the “Keep.” The entrance was over a drawbridge, and
through the embattled gate (Middle-age Latin balium or ballium,
a corruption of vallum, a rampart). When there were two courts
to a castle, they were distinguished as the outer and inner
bailey (rampart). Subsequently the word included the court and
all its buildings; and when the court was abolished, the term
was attached to the castle, as the Old Bailey (London) and the
Bailey (Oxford). Source: Dictionary of Phrase and Fable, E.
Cobham Brewer, 1894 More on Bailey from Infoplease: