The servants are all gone. So none of them will have seen the two men go into the cellars. Note the irony: “I had told them that I should not return until the morning…I well knew, to insure their immediate disappearance…as soon as my back was turned.”

The passage is extremely remote. “We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt….”

The nitre and the damp and the remoteness insure that few will willingly venture down here to discover anything amiss.

The revenge object is fettered, that is, chained to the wall and sealed up. The newly masoned opening is covered with old bones. No one will be able to hear a thing down here.

Montresor seems to suffer no guilt that might give him away. “My heart grew sick; it was the dampness of the catacombs that made it so.” You see, he is NOT sickened by his deed but only by the physical sickliness of the location. You know he will put on an innocent face to the enquiring world