How does Montresor hide his new construction?
Montresor has prepared the materials for making a wall across the front of the niche, which is only three feet wide and six or seven feet in height. He uses stones rather than bricks because he wants the wall to have a rough exterior resembling the natural granite walls of the catacombs. After he has completed his stone wall he uses the remaining mortar to plaster over the stones.
I forced the last stone into its position; I plastered it up. Against the new masonry I re-erected the old rampart of bones.
So the stone wall will look no different from the walls of the catacombs after the thick outer coating of mortar has dried. To hide his handiwork even further, Montresor erects a huge pile of human bones in front of it. No one would be likely to want to handle all those bones in order to look behind them.
What makes the stone wall safer from discovery is the fact that there is no apparent reason for any niche even being there.
Within the wall thus exposed by the displacing of the bones, we perceived a still interior crypt or recess, in depth about four feet, in width three, in height six or seven. It seemed to have been constructed for no especial use within itself, but formed merely the interval between two of the colossal supports of the roof of the catacombs, and was backed by one of their circumscribing walls of solid granite.
If anyone should want to inspect Montresor's premises after Fortunato disappeared, they would never notice any sign of a stone wall, especially since they wouldn't suspect the existence of any niche. It is extremely unlikely that anyone would want to look through the catacombs, because Montresor has taken such pains to make everyone think that he and Fortunato were the best of friends. Montresor has even conditioned himself to think of Fortunato as his friend. He refers to him throughout the narrative as "my friend," "my good friend," and "my poor friend." Montresor wants impunity, and he has thought things out far in advance.
The fact that his planning and his masonry are impeccable is shown by his conclusion of the narrative:
For the half of a century no mortal has disturbed them. In pace requiescat!
No doubt he has gotten rid of the trough, the trowel, and any leftover mortar and has been down there many times over the years to make sure the rampart of human bones has not been disturbed.