In a way, the entirety of the Commedia is an allegory; as Dante "literally" descends into Hell, through Purgatory, and into Heaven, he metaphorically descends into sin, goes through repentance, and enters Divine Love. But this allegory is largely situated in the specific allegory of Dante's pilgrimage and journey; like a pilgrim, he needs guidance from an experienced leader, in this case, the poet Virgil.
The Griffin (Symbol)
The Griffin, appearing in the final cantos (which are perhaps too full of symbolism to individually recount), is an important symbol of Christ himself. Just as the Griffin mixes two noble creatures, the eagle and the lion, Jesus mixes the noble aspects of the human and the divine; that he pulls the chariot which represents the Christian church only strengthens this symbolism.
The Eagle (Symbol)
The eagle, appearing in Dante's first dream, has classically been considered a symbol of Jove or Jupiter; yet Dante introduces two important differences. First, he transposes it to a Christian context, which was not entirely unusual; the Christian God was sometimes equated with Jove. But second, he makes the eagle represent Saint Lucy, introducing a feminine aspect rarely found in eagle symbolism.
Many of the souls in Purgatory are found singing hymns; this form of song represents their spiritual devotion, and the content of the hymns often reflects their own penitential goals. Yet music is also connected to Dante's poetry: there is some sweet music he enjoys, but there is also seductive and false music, like that of the woman in his second dream. Music becomes a complex motif, mixing both devotion and temptation in one.
The Steps (Allegory)
The three steps into Purgatory, which are white, purple, and red, are an allegory for the three stages of absolution. Respectively, they represent confession, contrition, and satisfaction, and Dante imbues them with a number of suggestive details to turn what could be a simplistic allegory into an intense and wonderfully described scene.
Divine Comedy: Purgatorio Questions and Answers
The Question and Answer section for Divine Comedy: Purgatorio is a great
resource to ask questions, find answers, and discuss the novel.