The most significant allegory in "The Cask of Amontillado" is the cask itself. The titular cask represents an undefined ideal, never seen in the story but coveted by Fortunato, and it is the thing that leads to his death. When Montresor shackles Fortunato to the walls, the following exchange occurs:
"Pass your hand," I said, "over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power."
"The Amontillado!" ejaculated my friend, not yet recovered from his astonishment.
"True," I replied; "the Amontillado."
(Poe, "The Cask of Amontillado," eNotes eText)
This is the crux of Montresor's plan, and it seems to occur to Fortunato for the first time that simply speaking of a cask of Amontillado does not make it real. The cask is what Fortunato seeks, but is in fact his own death at Montresor's hands; for Montresor, meanwhile, the cask represents nothing more than a lie told for his plan. To Fortunato, however, the cask was something to strive for, something to show off his palate, and something to brag about in company. This is his undoing; in pursuit of an unattainable ideal, he falls prey to Montresor's revenge.