The Raven is a Wicked Bird
The rest of the song is explaining how he ended up in his prison. It doesn't describe the prison walls. It doesn't describe the Raven itself. Only a dark haired woman, bathing in a creek. And his love.
Ravens remember everything, everyone who hurts them, everyone who helps them, and who to trust. When we meet ravens, they fill us with dread, because they eat the dead. They appear when we are weak. They are a good omen, and a bad omen.
Is the Raven truly a wicked bird? The Man killed Kate McCannon. The woman he loved, he slayed her because he found looking at someone the same way she looked at him. He knew who she was, and loved her. But he didn't see her. His sin was that he loved without knowing. And he killed his love.
Kate McCannon was simply doing what Kate McCannon does: bathe in the creek. The Man thought that he could change her, and he did. He killed her.