What detail of the setting seems especially appropriate to the plot?"The Cask of Amontillado" by Edgar Allan Poe

Most significantly, that is it the Carnival season for
Poe's setting gives rise tothe masquerade, recalling the Restoration Period of England
in which the aristocracy disguised themselves and partied with those of the lower
classes. Disguised, these people entertained their more prurient desires without
reproach. Likewise, Montresor entertains his horrifying reprisals against Fortunatoby
means of guise within the setting of the Carnival, a time of promiscuity, revelries, and
debauchery. 

Interestingly, however, Poe turns the
setting of the masque into his classic arabesque that twists and subverts the plot as
much of the guise is what actually is apparent, not what is hidden.  For instance,
Fortunato is disguised as a harlequin, or fool, when he really is foolish so he should
not wear a disguise for what he already is. Another example occurs when Fortunato asks
Montresor, "You are not of the masons?"  and Montresor ironically replies, "Yes, yes"
playing upon the double entendre of the word mason. In another
masque Montresor, an Italian, presents the family coat of arms to Fortunato, but it is
disguised by the motto of the royal arms of Scotland which indicates Montesor's deadly
intentions. 

Certainly Montesor,
under the appearance of disguise gives Fortunato several hints of his intentions, but
Fortunato is too crass to comprehend them.  So, the real disguise in "The Cask of
Amontillado" is the reality of Poe's horrific deed done during the time of the masque
and the Carnival; a punishment with impunity.