There are frequent instances of grimly ironic humour, where Montresor says or does something and Fortunato actually agrees with him, never comprehending the far more sinister significance of what Montresor is actually saying. It is this aspect that, to my mind, actually makes Montresor far crueller as a character, as not only is he going to carry out a pre-meditated murder, but he is deliberately toying with Fortunato as they venture down into the catacombs. One example is when Montresor tells Fortunato the motto of his family, which can be translated as: "Nobody attacks me without punishment." What is ironic is the way that Fortunato responds:
"Good!" he said.
Unknowingly, he is applauding the thirst for revenge that will lead to his own death.
Another episode that is my favourite comes just a moment afterwards, when Fortunato tests to see if Montresor is a Mason with a secret gesture. Although it is clear that Montresor is not a member of the Masons, a secret society, Montresor deliberately twists what Fortunato is saying, insisting that he is a Mason, and showing the trowel as proof. Again, Fortunato misunderstands, saying "You jest" in response, blind to the terrible irony. For he will soon understand that Montresor is in fact a Mason - as demonstrated in his skill of building the wall to seal Fortunato away from the rest of the world forever.
So, while there are elements of humour in this tale, they are grimly and horrendously ironic, and serve to heighten the macabre atmosphere in this tale by revealing more of Montresor's cruelty.