While the narrator's friend was chained, what was Montressor doing in the Edgar Allan Poe short story, "The Cask of Amontillado"?

    While you could hardly call them "friends" at this point in the story, Montressor kept himself busy after chaining Fortunato to the pilings in the Edgar Allan Poe short story, "The Cask of Amontillado." After luring Fortunato to "the extremity of the niche," Montressor made his move. Before Fortunato knew what was happening,

... I had fettered him to the granite. In its surface were two iron staples, distant from each other about two feet, horizontally. From one of these depended a short chain, from the other a padlock. Throwing the links about his waist, it was but the work of a few seconds to secure it.

Montressor stood back and sarcastically asked Fortunato if he would now not like to turn back. "The Amontillado," Fortunato implored.
    But there was no Amontillado. As Fortunato stood bewildered at his unexpected predicament,

I busied myself among the pile of bones of which I have before spoken. Throwing them aside, I soon uncovered a quantity of building stone and mortar. With these materials and with the aid of my trowel, I began vigorously to wall up the entrance of the niche.

Brick by brick, tier by tier, Montressor constructed a fourth wall that would inevitably seal his "friend" inside forever. After nearly finishing, Montressor paused to peer inside. Fortunato laughed.

    "Ha! ha! ha!--he! he!--a very good joke indeed-- an excellent jest. We will have many a rich laugh about it at the palazzo--he! he! he!--over our wine-- he! he! he!"
    "The Amontillado!" I said.
    "He! he! he!--he! he! he!--yes, the Amontillado. But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone."
    "Yes," I said "let us be gone."
    "For the love of God, Montressor."
    "Yes, for the love of God."

And with that, Montressor completed his task, and Fortunato was heard no more.