CS Lewis specifically noted that Narniais NOT an allegory, and as a professor and chair of medieval and rennaissance literature at both Oxford and Cambridge, he ought to know.

"If Aslan represented the immaterial Deity in the same way in which Giant Despair represents Despair, he would be an allegorical figure. In reality however he is an invention giving an imaginary answer to the question, ‘What might Christ become like, if there really were a world like Narnia and He chose to be incarnate and die and rise again in that world as He actually has done in ours?’ This is not allegory at all."

- C.S. Lewis

From: Letters of C.S. Lewis