Why does Montresor keep urging Fortunato to turn back in "The Cask of Amontillado"?
The is actually a ploy designed to appeal to Fortunato's pride. Whenever Montresor urges Fortunato to turn back, he also adds, "Besides, there is always Luchesi". Since Fortunato is so proud of his ability to judge fine wine, Montresor knows he will not allow Luchesi to judge the wine. In fact, Fortunato always adds, " Luchesi cannot tell Amontillado from sherry." So Montresor lures Fortunato farther and farther into his vaults using Fortunato's pride, not really the wine, to get him where he wants him to be.
Luring Fortunato into the catacombs is Montresor's cat-and-mouse game where he plays with his prey before 'doing him in.' It also heightens the aspect of perfidity in Montresor, who has obviously taken pleasure in leading Fortunado on. Remember in the opening paragraph he hints that this is part of the gratification of vengeance. In short, it is his kind of sordid 'fun.'
Another intrinsic metaphor here: Note that the nitre forming in the Montresor catacombs has made the configuration of a spider's web. Montresor leads Fortunato to its centre much as a spider would "reel in" an insect already snared by its web.