What are the literary devices used in "La Belle Dame Sans Merci" by John Keats?

The main device used throughout the poem is repetition, which is characteristic of the traditional form the poem takes: the ballad. Ballads pertain to the oral tradition and are designed to be told or sung: repetition is a way to keep listeners engaged and even to make them participate in the telling. The fact that the poem is written in a rather simple and antiquated language also anchors it in the tradition of the folktale.
Throughout the poem, silence and death are intertwined through imagery: "no birds sing," "wither'd plants" run parallel to the description of the knight, "alone and palely loitering."
Keats uses a few metaphors that reinforce the lyricism of the poem and the otherworldliness of the setting. For instance, the pallor of the knight becomes "a lily on [his] brow" in the poet's description. This could be a hint at how the knight is under the spell of the lady and risks losing himself in her world, since she is associated with flowers all through the poem.
These are seeds you can use in your analysis. Try to spot patterns in sound and imagery in the text: what kind is associated with whom, and to what effect?