How is The Faerie Queene enjoyable as an adventure and allegory?
If you can follow the action, amid all of Spenser's poetic and archaic language, the adventures of Red Cross and Una are exciting. While so much of the poem is an allegory (that is, a character, action, or name which represents something else --- usually an idea, virtue, or tenet of faith) there is still a thread of narrative action which Spenser keeps going. There are many perils and ordeals that Una and Red Cross go through, and, while Spenser often goes off on tangents to explain his religious worldview, the story is still there.
The drama of Book I is perhaps the most intense -- the menace of the dragon, and the peril that Una's parents are in, provide the initial motivation to set out on a journey for Una and Red Cross. The horrible Error, though described sometimes in difficult language, is a very real threat in Spenser's mind. Since the monster is so named Error, it is difficult for the reader to think of it other than its allegorical meaning (the actual errors of faith which human beings are prone to making.) This is in direct controversion to what most people in the modern age are taught about error (in faith, or in intellectual or spiritual pursuits.) Error is often thought today as a learning experience -- a chance to correct one's mistakes and grow from the experience. It is a good exercise for the modern reader to understand that Spenser, and many people in his time, did not see religious error in this light. For Spenser, any error in faith, even if it was only temporary, was a direct imperilment to a person's soul. In other words, Error (and error) were to be avoided at all costs, as something inherently toxic and unable to bring about any good whatsoever. Once the reader grasps this fundamental difference between much of modern teaching and Spenser's ideas, some of the main ideas of The Faerie Queene become easier to understand. This is an example of how the allegory becomes the star of the poem, rather than the narrative (as was, doubtless, Spenser's intention.) The book ends, with the narrative becoming exciting again, with the fairy-tale ending of the dragon being slain, the hero's parentage revealed, and a wedding between the hero and the beautiful princess (Una).
Book II is very thick with allegory, the upshot being that Despair is one of the most horrible sins, which can lead to an even worse sin (a mortal one) -- suicide. Again, the difference between the modern view and Spenser's is highlighted; while modern people certainly agree that suicide is horrible and to be avoided, the shame on the family of the suicide is not what it was in Spenser's time. More importantly, however, Spenser believed that the suicide was forever damned to hell; therefore Despair is every bit a danger as Error.
Books III and IV become more exciting from a narrative point of view, with the introduction of the virgin knight Britomart and her many adventures. Some of the allegory is lifted in these books, and the fascinating and evil Duessa is an interesting addition.
Allegory returns in full-force (as it never really leaves the narrative of this epic poem) in the last two books of the poem. The characters of Calidore and Arthur are often thought not as compelling as Una, Red Cross, and Britomart, so the reader cares less about their adventures than in earlier books. The allegories of justice and civilized behavior are clear, however, and support the Elizabethan ideals of high culture.