Montresor's main problem is to lure Fortunato off the streets and down into the catacombs beneath his palazzo. The streets are crowded with revelers in costumes. Most of them are intoxicated, including Fortunato. Montresor wants to lead him away without anyone noticing him. It doesn't matter if they notice Fortunato as long as they don't notice who is with him. Here is a pertinent paragraph:
It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend. He accosted me with excessive warmth, for he had been drinking much. The man wore motley. He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells. I was so pleased to see him that I thought I should never have done wringing his hand.
The words "supreme madness" do much to make the reader visualize the noisy, colorful scene. Poe shows his genius here. Montresor is able to escape notice because the man he is with is so conspicuous. Fortunato is wearing a jester's costume complete with a cap and ringing bells. He is a man who loves to be the center of attention. Montresor is wearing a black cape and a black mask. He is like a shadow. Tomorrow morning when inquiries are starting to be made, many people will remember seeing Fortunato but no one will remember seeing anyone with him. The most important aspect of this part of the story is the crowds of noisy, drunken people from among whom Montresor must pluck his victim and lead him to his doom.