It is clear that in this masterful short story, Poe creates an atmosphere and mood of terror and fear. Certainly there are plenty of ways in which he does this, but you might want to consider a description from the middle of the story, whilst the narrator is in the room of the old man and is contemplating his eye, waiting to kill him. Suddenly, he is able to hear the heartbeat of the old man, thanks to the acuteness of his senses he has:

Meaning the hellish tattoo of the heart increased. It grew quicker and quicker, and louder and louder every instant. The old man's terror must have been extreme! It grew louder, I say, louder every moment!

Consider how the description of the "hellish tattoo" of the old man's heart that the narrator is able to hear, and how it becomes louder and quicker every second, helps raise suspense for what is going to happen and also conveys the severe terror of the old man, who knows that someone is in the room with him and is terrified. Thus the mood of fear and terror is sustained in this section of the short story.