Allegory in the Middle Ages was a vital element in the synthesis of biblical and classical traditions into what would become recognizable as medieval culture. People of the Middle Ages consciously drew from the cultural legacies of the ancient world in shaping their institutions and ideas, and so allegory in medieval literature and medieval art was a prime mover for the synthesis and transformational continuity between the ancient world and the “new” Christian world.
Debora B Schwartz Practical definitions and examples of medieval allegories.
Debra Bruch Abstract from academic paper with brief but useful section discussing allegory as a form of Medieval drama (look under sub-heading ‘Transition to Defense’).
OldAndSold Article discussing allegorical comedies in 16th century England.
Nellen Homepage Good, brief definition of allegory.
University of North Carolina Definition of allegory.
Wikipedia Various definitions of the term allegory and its uses in literature, the arts etc.
WiseGeek Extended definition of allegory with examples.