There are several uses of symbolism in this story.
The carnival: Montresor refers to 'the supreme madness of the carnival season' during which the story takes place, and which may represent his own madness. The carnival is traditionally a time of celebration but also can be wild and unruly - like Montresor's thoughts and plans. At the same time, he disguises these unholy feelings before his intended victim, Fortunato. Carnivals also involve disguises.
Catacombs: these grim dark underground vaults might be taken as symbolic of the evil in Montresor.
Fortunato's jester outfit: This symbolises the foolishness of its wearer in letting himself be lured to his death by Montresor, whom he has injured in the past, and therefore should have been a bit more wary of. (It should be added, though, that we don't actually know what has happened between the two men to make Montresor lust for revenge; it may have been something quite slight, and maybe Fortunato is not even aware of it.)
Fortunato's name: which means the fortunate one, can be regarded as an ironic symbol as of course Fortunato ends up being anything but fortunate.
Montresor's family crest which bears the motto: "Nemo me impune lacessit": This means: 'No-one injures me and gets away with it.' This is borne out by Fortunato's grim fate. The family emblem also symbolises what happens between Montresor and Fortunato: it shows a foot crushing a serpent which is biting the foot. The serpent represents Fortunato, who has injured Montresor; the foot stamping down on the serpent represents Montresor's revenge.