What role does allegory play in the Augustinian view of history?

Augustine--despite his best efforts--is 'credited' with the development of the doctrine of Original Sin, in which allegory plays such a central role that the Councils of Trent were unable to even define Original Sin (v. the Law of sin and death), or say exactly what it meant; even though it was ratified, anyway (for obviously political and not spiritual reasons), much to Augustine's chagrin, or so history tells us. See the link for an overview.