The trowel is a symbol of revenge. It is proof that Montresor thought the plan carefully through. The cask is a symbol of gluttony. Fortunado suspected nothing and only cared about the wine. Finally, the crypt is a symbol of the darkness of mans' soul.
What about the coat of arms that represents the Montesor family? The logo is the motto of Montesor: "No one attacks me without punishment," and the representation on the coat of arms is that of a huge golden foot crushing a serpent against a blue background.
Another symbolic item is the costume of Fortunato: the harlequin of a fool, or the jester. Clearly, Montesor, who finds this costume the perfect outfit, delights in pointing Fortunato's apparel out to the reader:
The man wore motley. He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells. I was so pleased to see him....
Just to develop #4 a little - I also think that the catacombs are a symbol of Montresor and his twisted, manic personality. Clearly he is a character that has a strongly defined "public", acceptable personality and then through his first person narrative we are exposed to his chilling, homicidal "sub" personality that the catacombs, in their labyrinthine and twisted fashion, exemplify. Likewise #5 is key in highlighting the use of setting. Carnival is also a time of disguise, of playing with different identities and dressing up. It is highly symbolic (and perhaps ironic) that Montresor chooses this time to reveal who he really is.
How about the fact that the entire incident takes place during Carnival? This is a time of feasting and revelry and celebration, as alluded to above. It's also a time of lowered inhibitions, a time when people do things which they may not do during other times. That is, of course, exactly why Montressor chooses this time; he knows Fortunato would probably never be so willing to go so far (and in such a rather drunken condition) as he would during the days of Carnival. The Carnival, then, is symbolic of careless behavior with minimal consequences.
I think the catacombs serve as a symbol of Montresor's evil thoughts: just below the happy, celebratory streets are the catacombs, dark and eerie. Similarly, Montresor seems to be a well-to-do and well-mannered member of society, but he also has a dark and eerie personality just below the surface. Both the catacombs and Montresor's true personality cannot be seen from the "surface", but they are both dark and dangerous once discovered.
I have always enjoyed Poe's pun about the trowel and its relationship to the Freemasons. This occurs when Fortunato asks Montressor if he is a "member of the brotherhood." When Montressor says that he is, Fortunato asks for a sign: Montressor produces the trowel. Fortunato does not understand the irony, but the trowel serves as the symbolic weapon of his death. It is the tool that Montressor uses to construct the wall that will hide Fortunato's body for the decades to come.
It's been a long time since I've read or taught this story, but one symbol you can definitely count is the cask of the Amontillado wine, itself. It is the sole reason Fortunato has agreed to follow Montressor into the dank and depressing dungeon decorated with bones in the corners. All of these things should have been a huge red flag, but the wine is a rare and special type of which symbolizes to both parties the success and wealth of the owner of such a possession. Montressor knows this, and uses this knowledge as bait to capture his rival's attention so that he could literally capture his rival behind a bricked-in grave.
It shows how people today go so far to get what they want.