Novels can be allegorical. For a novel to be allegorical, the characters, setting, storyline -- in short, everything that makes a novel a novel -- must have direct equivalents in a real-world phenomenon, historical event or situation. For example, in George Orwell's 1945 novel "Animal Farm," the animals who take over the farm, as well as their hierarchy, interactions and decisions, represent the events leading up to the Stalin era before World War II. Writing an allegorical novel does not require any specific writing style -- the novel can be in first or third person, it can be narrated in the present or past tense, and there are no unusual restrictions regarding the narrative structure or syntax. In order for the allegory to work, however, it's important that the extended metaphor include every aspect of the novel, so that the reader is shown rather than told the writer's opinion and interpretation of the allegorical object.
Poetry is also an excellent medium for constructing an allegory. Poetry differs from prose in novels and short fiction because it has more restrictions regarding structure, rhyming, rhythm and form. Classical poetry includes established poetic forms, such as the sonnet or pantoum, as well as set meters and rhyme schemes, which give poetry specific melodic and rhythmic qualities that are not commonly found in novels and short fiction. However, poetry can also be composed in free verse, a format that has no rhythmic, rhyming, or structural limitations. An allegorical poem, like a novel, should tell a story, or describe the point of view of a person or character experiencing an aspect of the writer's chosen metaphor that represents a larger phenomenon. The poets Thomas Hood and Stephen Dunn wrote allegorical poems that they signaled by including "allegory" in the titles, such as Hood's "Allegory" and Dunn's "Allegory of the Cave," a response to Plato.
Drama is another writing style that can be used to create allegory. Drama differs from novels, poetry and other forms of prose because the writer only has unspoken stage directions and spoken dialogue among characters to convey an appropriate metaphor to the audience. This makes drama a particularly challenging writing style to master. The major advantage of writing a play, however, lies in the dramaturge's capacity to construct an entire world using the stage and props -- it's much easier to understand an extended metaphor with so many visual and auditory cues.
Samuel Clemens (Mark Twain) once apologized for writing a long letter, claiming he did not have sufficient time to write a short one. That paradox illustrates that challenge of successfully completing an allegorical work of short fiction. The short story is one of the hardest genres for any writer to master. As with a novel, a short story also requires a beginning, middle and end, but in far fewer words. Because of its brevity, the short story is an excellent format for spinning an allegorical tale. Writers of short stories frequently choose to limit the scope of the story to the events of a dinner party, or a conversation in a restaurant, rather than trying to capture the broad scope of a novel in such limited space. The dinner party of restaurant setting becomes the setting that will carry the allegory, and everything the characters do and say can develop the allegory further.
A piece of writing doesn't always need to be completely allegorical. Writers can also follow Plato's example and insert examples of allegory in their piece of writing, as a story a character tells, or a lesson one character teaches another. This sort of brief example of allegory operates on two levels -- on the first, the character describes the allegory and another character interprets the metaphor; and on the second, the reader interprets the allegory as the characters present and analyze it.