Keats evokes several landscapes within the poem: the desolate setting of the opening ('the sedge is withered from the lake'), where the unnamed speaker of the poem first addresses the knight; the idealised supernatural landscapes of his encounter with the lady ('honey wild, and manna dew'); and the nightmare, Gothic vision of her abandoned former lovers ('starved lips in the gloam'). These last prompt the abrupt transition back to the first, and the knight's answer to the question.
In Romantic fashion, Keats is toying with conventions of medieval romance, but he uses it to explore new ideas, specifically those of the imagination. The knight's experience with the 'belle dame' is rich in ambiguity, but certainly has many of the qualities of dream. Given that the knight also falls asleep, and dreams, during his encounter with the lady, the Gothic nightmare landscape becomes further removed from the 'reality' of the framing narrative. Keats shows us the transformative powers of the imagination through the knight's experience, perhaps suggesting the Muse-like qualities of the 'belle dame', and hence his desolation when abandoned by her at the poem's close.
Keats' "lady without mercy" is described as the child of a fairy in the poem. Right away we know that this poem is going to be chock-full of supernatural elements, but also, to coincide with folklore and tales, the nature aspect of this poem is inevitable. Fairies are attached to the landscape...wood nymphs, water sylphs, flowers...are all examples of fairy existence you will find in literature.
The knight falls in love with the child of the fairy, and for a time, things are great. They exchange gifts made of the natural surroundings, and he stays with her in her cave on the side of the hill. While they are together, everything is as spring and summer--full of life, full of beauty, full of joy. However, when the knight awakens to find the lady gone, the images are not of spring and summer, but more like late fall and winter. The animals have stored food for the cold, the grass is dying, the birds are quiet. Did he imagine her?
Another speaker asks about a lily on the knight's brow--a lovely flower, and a sign that the lady has been there, but it is also symbolic of death. Life as the knight has known it is ending and dying like the signs of nature surrounding him. Imagined or not, he can not live without his love, similar to the kings in his dream who warn him of her charms. His cheeks also show the rosy glow of one recently touched by love and beauty, but like the other signs of nature, that color is also fading.