One curious thing about this episode is the fact that Montresor happens to have a trowel under his cloak. Was he expecting to meet Fortunato that evening? If so, why? Or did he always carry the trowel under his cloak in case he should meet Fortunato and be able to lure him back to his palazzo?
Montresor had the building materials prepared in front of the niche where he planned to wall up Fortunato, but evidently he didn't want to leave the trowel with the stones and mortar because, as he repeatedly pointed out to Fortunato, it was extremely damp down there. They were actually under a river, and water kept dripping down on the bones that were piled over the stones and mortar. This was necessary for the plot because the water kept the mortar fresh. Otherwise it would have turned into something as hard as cement. Poe wanted to expedite the action and not have to make Montresor mix mortar. He couldn't leave the trowel with the building materials because it would have rusted. Poe didn't want to have him leave it in his house because there would have been an awkward time lag while he went to get his trowel.
The wall is constructed with surprising quickness. Poe interrupts the building process to describe Fortunato's actions inside the niche, his screams, his pleas, etc., in order to avoid a long, dull description of building a stone wall--which in reality should take a couple of hours, especially since Montresor could not be an experienced stone mason.