What words in the poem build up the uncanny atmosphere?  The word that describe the forest and the horse are not particularly uncanny. They seem to be quite normal.

It is the words that describe the house, the listeners, and the behavior and feelings of the Traveller that lead to the atmosphere of the poem. For example, "he smote upon the door" leads the reader to a different feeling than if he had simply "knocked upon the door." What does the word "smote" mean?  Check your dictionary to see. The scene is "moonlit," which also contributes to the atmosphere of the poem.  Could it be as effective if the action took place in sunlight?  Probably not!  The house has a window sill described as "leaf-fringed," which suggest to the reader that the house was deserted long ago.  Who would live in a house that had vines growing up over the window sill? 

As you read through the poem, you will see any number of words that contribute to the eerie feeling of the poem. Here are just a few: dark, empty, lonely, strangeness, and stillness.   Have fun finding the rest yourself.

Good luck to you!