Deep into that darkness peering, long I stood there wondering, fearing, (line 25)
The natural world is definitely part of what makes this all so spooky. There's this one safe, warm little room, surrounded by a big scary outside world. In lines like this one, we can almost feel the outside pressing in. When the speaker opens the door into the darkness, he nearly gets lost in it for a moment, hypnotized by the mystery of what might be out there. He's always looking for clues in the natural world, trying to learn the answers to impossible questions. As hard as he looks into the darkness, he doesn't see a thing, and when he hears a whispered word, it's only his own voice being echoed back.
'Tis the wind and nothing more!" (line 36)
Faced with a scary, mysterious natural world, the speaker tries to comfort himself by coming up with a logical explanation. Have you ever lain awake in bed at night and heard a sound from outside? If it starts to freak you out, what do you say to comfort yourself? Probably something like what our speaker says: you try to imagine a normal, non-scary source for the sound. To do this, you have to convince yourself that the natural world outside isn't evil and out to get you. Instead, it's just a place where things happen randomly, and where every now and then things make unexpected noises. Good luck, though, if you happen to be a character in a Poe story, because when you hear a strange noise, you know by now that you're in real trouble.
In there stepped a stately Raven of the saintly days of yore; (line 38)
If this struggle between man and nature were a boxing match, our speaker's opponent would have to be…the Raven. This creepy bird symbolizes the intrusion of nature into the speaker's safe little world. He's also full of all the mystery that's out there in nature. It might be that he's pure evil. It might also be that he's just a bird, and a "saintly" one at that. Whatever the truth, this doesn't turn out to be a fair fight. The Raven only has one punch, but it knocks our poor speaker out pretty fast.