That saltpeter crust on the walls is the final, perfect detail that seals the entire revenge. Montresor isn't just walling Fortunato up; he’s entombing him in a place that, on the surface, looks like a legitimate wine cellar. The nitre, which Fortunato even comments on earlier, sells the illusion. It’s a sign of dampness, of a place where fine wine might be stored, which is why Fortunato never questions being led deeper. By the climax, that same damp mineral growth becomes a visceral part of the horror—it’s what he’s breathing, what’s probably cold and slimy on the walls as the last brick goes in. It transforms the space from a simple recess into a literal catacomb, connecting Montresor’s private crime to the ancient, dusty death of the family vaults. The importance isn't chemical; it’s atmospheric and symbolic, making the setting feel authentically, historically dead.
Some folks focus on the amontillado itself as the lure, but the nitre is the environmental lure. It appeals to Fortunato’s pride as a connoisseur—'the nitre!' he exclaims, as if noticing a feature of a fine cellar. That misplaced expertise is what dooms him. In the climax, as he’s silent and Montresor is working, the nitre is the only described feature of the chamber. It’s the last thing his eyes might see, glittering in the torchlight, a false promise of something valuable in a place that only holds his end. Poe doesn’t need to describe it again at the very end; its presence has already been established as the wallpaper of the tomb.