Both "The Tell-Tale Heart" and "The Masque of the Red Death" employ timepieces (watches or clocks) as symbols. In "The Tell-Tale Heart," the narrator believes that he hears an old man's heart beating, a sound he compares to the sound a watch makes when it is wrapped in cotton. However, it is not the old man's heart the narrator hears, but his own, and the sound enrages and frightens him because it sounds like a timepiece, symbolic of his own mortality and finite lifetime. In "The Masque of the Red Death," an ebony clock stands in the final room of seven that runs from east to west, like the sun's own movement. The room is red and black (black, itself, is another popular Poe symbol -- we see it in "The Raven" with the black raven and "The Black Cat" with the titular cats), and it too symbolizes death. The clock, here, also symbolizes mortality and reminds people of their own eventual deaths, which are inescapable.
Eyes are also a symbol in several stories. In "The Tell-Tale Heart," the narrator wants to kill an old man because of the man's "vulture" eye, which likely has cataracts (as this disease would account for its cloudy appearance and his apparent blindness in that eye). Though the old man seems not to be able to literally see with this eye, he seems to have a kind of second sight, as, when the narrator makes a noise in the old man's room one night, the old man cannot go back to sleep and utters a "groan of mortal terror"; if he trusted the narrator, why would he be so frightened? Further, in "The Black Cat," the narrator cuts out the eye of his first cat, named Pluto (the Roman god of death, and another symbol of death, as the narrator in "The Raven" also alludes to), because that cat seems to understand the narrator's cruel nature. Then, the narrator finds another black cat, but this cat is already missing an eye, another symbol: it cannot physically see the narrator from this eye, but it can figuratively see the truth about the man.
The idea of murdering someone and hiding them within one's home also seems to symbolize one's guilt. The narrator of "The Tell-Tale Heart" murders the old man and buries his body beneath the floorboards. The narrator of "The Cask of Amontillado," named Montresor, walls his enemy, Fortunato, up in the Montresor family's catacombs beneath his house. The narrator of "The Black Cat" kills his wife and walls her up in his basement. The first narrator's guilt gets to him, making his heart race (a sound that terrifies him), and he confesses to the murder. The second narrator believes that he has gotten away with his crime, but there is evidence to suggest that his guilt catches up with him in the end. The third narrator gets cocky and raps on the wall with his cane when the cops come to investigate his wife's disappearance, and when they hear the cat's scream, he learns that he's walled the cat in with his wife's corpse. Nobody really totally gets away scot-free; at the very least, their guilt makes them pay for what they did, and it sometimes forces them to confess.