Edgar Allan Poe has a reputation for giving us rather insane narrators, who quite understandably give us more than just a little cause for doubt when it comes to the truth of their accounts. They often make frequent claims as to their continued sanity, however, which makes their accounts even less believable.
So when we first begin reading The Cask of Amontillado, we will, if we have had at least a passing familiarity with Poe's style, immediately suspect Montresor of being as unreliable as all of Poe's other narrators. But a close re-reading of the story shows things to be rather different, if we are willing to consider that Montresor is not an unreliable narrator. By reading The Cask of Amontillado another time, preferably immediately after reading it the first time, some of the details begin to take a very different shape, now that we can place them in context of the entire story, and not merely what we have read up to that point.
To begin with, it appears that Montresor is making a sort of confession on his deathbed, something which should be taken apart into its two halves— "confession" and "on his deathbed"— so that we might examine them separately.
It will be easier to prove the latter half first. At the end of the story, Montresor says that Fortunato's bones have lain in the crypt for half of a century. This need not be an exact timeframe, and could conceivably be anywhere from forty to sixty years. Montresor, at the very least, has to have been twenty years old at the time that his tale takes place— not to be confused with the time when he tells this tale. If we decide that his usage of "half of a century" is to be taken as meaning forty years or so, then he is about sixty years old when he is narrating this story. For the times, this is not all that young, and it is not inconceivable that Montresor is soon to die at this age. He is, however, likely older, and it is quite likely that he is in his mid-thirties— we might do well to remember that Fortunato needs time in which to commit the "thousand injuries" of which Montresor speaks.
It is, in fact, this point which makes it appear as if Montresor's account is a confession. A confession does not have to be the disclosure of one's wrongdoing in the hopes of receiving an absolution. A confession is simply an open admittance of one's deeds, whether they be wrong or not. But Montresor certainly seems to be confiding some story, one which may very well have been weighing on his conscience for some time— while he discounts the sickness in his heart as being on account of the damp, it seems most probable that this belief is merely the result of some forty-to-sixty years of rationalization.
It is in the beginning of his tale that he refers directly to the audience, and speaks not only to the reader, but of him (or, far more likely, of her). The implied audience, of course, is just as important to understanding a story as is the narrator, and the story itself, and there are some very strong impressions which are acquired when Montresor says "You, who so well know the nature of my soul…" It appears that the implied audience is one who is very familiar with Montresor and the ways of his mind, and I say that I am not mistaking vicious zoogs for harmless dust bunnies beneath my bed, when I say that this, combined with his likely age, leaves only one truly possible conclusion.
He is not simply speaking to any random person. Montresor is speaking to someone with whom he is very intimate. Physically, most likely, but, more importantly, emotionally, psychologically, and mentally. He is speaking to his wife, who, it might be noted, is most likely far younger than him, and still possessing many years of ahead of her. It was not all too rare for women to marry men older than themselves, and if Montresor had had a wife at the time of his story, he surely would have mentioned it when he said that he had made sure that all his servants had departed out from his house.
So he is making a confession, and doing so on his deathbed, and to his wife. To his confidant, I might say. It does not strike me as being plausible that she has not seen her share of the inner workings of his mind, and all his many worries, and his secret desires. So there is no reason for him to decide, for some odd reason, to tell her this story, and then lie about it. He is already a murderer, and it seems, from the way he talks to her, that the idea of him killing Fortunato is not even all that surprising, so what could possibly be worse?
Before closing this essay, it might also be useful to note the context of the story, as well, which provides one final reason to suspect in the truthfulness, sanity, and, above all, reliability of Montresor. There is a very good possibility that Montresor was an author avatar for Poe, with The Cask of Amontillado being nothing more a bit of a combination of a "take that," and a self-insert fic. For quite a long time, Poe had had a bitter rivalry with Thomas Dunn English, an author and songwriter. They had traded back several works deprecating the other, turning the other into a caricature— and usually visiting upon them some unpleasant fate. Eventually, this would lead to one of English's caricatures of Poe going too far, and Poe suing for libel. In it, Poe had been portrayed as a drunken, womanizing liar, and a wife beater.
This latest work, titled 1844, or, The Power of the S.F., had several different details later referenced in The Cask of Amontillado, which was published soon after. Fortunato makes an unnecessary reference to the Masons, one which could have been taken out of the story without hurting it at all, and the gesture which he makes is similar to the one used in the secret society in 1844. In English's book, there was the image of a eagle, with a snake grasped in its claw, which is quite reminiscent of Montresor's coat of arms, in which a human foot is stomping upon a snake, which is biting the foot in the heel.
It is, thus, quite easy to see Fortunato as being representative of Thomas Dunn English. Fortunato has delivered barb after barb, cut after cut, and given Montresor— or Poe, rather— a thousand little injuries over the years. The fact that Fortunato seems to have a quite amicable relationship with Poe might also be a reference to the old— and very much dead, at this point— friendship once held between English and Poe. Similarly, the idea that Poe is Montresor is quite an easily defensible one, making The Cask of Amontillado something of a written fantasy, a bit of self-indulgence into what Poe may have been wishing to do to his old ex-friend at this point— Poe's reputation was indeed taking no light beating, the same as English's.
If so, it would hardly be sensible to claim that Montresor is insane, or lying. To say that Montresor is insane would be to say that Poe is insane, and it is very unlikely that Poe would want to cast such an image. It was this whole mess of being portrayed as a lunatic— or worse— that inspired him to write The Cask of Amontillado. Similarly, for Montresor to be lying would, it seems, the purpose of the story would be defeated. While the context of the story and the inspiration behind its creation is not really, by itself, enough to conclude that Montresor was sane, and telling the truth, and doing so as accurately as is possible for him at the moment, it would appear that these things, in combination with the clues placed within the story, and which stand on their own readily enough, make this conclusion impossible to defeat without some sort of massively crucial yet overlooked detail.
Due to his status as Poe's author avatar, it might actually be concluded that even the fog of the ages has had no effect on Montresor, even in his old age, since an unreliability stemming from a bit of fogginess in Montresor's memories would defeat the story's purpose just as quickly as a bit of fibbing or exaggeration on the man's part.