Where It All Goes Down
From beginning to end, you feel like you're in some kind of medieval fairy-tale when you read The Eve of St. Agnes. You've got chilly castles, ancient blood-feuds, nursemaids, forbidden love—if anything, Keats overdoes it. Just check out all of stanza 24, which is devoted entirely to one window:
A casement high and triple-arch'd there was,
All garlanded with carven imag'ries
Of fruits, and flowers, and bunches of knot-grass,
And diamonded with panes of quaint device,
Innumerable of stains and splendid dyes,
As are the tiger-moth's deep-damask'd wings;
And in the midst, 'mong thousand heraldries,
And twilight saints, and dim emblazonings,
A shielded scutcheon blush'd with blood of queens and kings. (208-216)
As this level of detail shows, the poem goes to great lengths to set things up in a gothic castle, steeped in mentions of warring families and enchantment.
Something really interesting in this poem is all of the stuff that Keats obviously isn't telling you, though. You feel like you know every nook and cranny of Madeline's bedroom by the end of the poem, but where do these people live? When? Your sense of where and when everything is taking place is both really strong and, at the same time, really vague.
So, to sum up: you've got the plot set-up of Romeo and Juliet playing out in a vaguely medieval landscape, inside a castle that houses people with names that belong everywhere from ancient Greece (Porphyro) to Medieval Germany (Hildebrand). Keats alludes multiple times to the fact that the families of Porphyro and Madeline have major beef with one another, but he never actually tells us what the problem is. We have no idea how these two kids even got together, or what their relationship is like. Instead, Keats gets to rely on the Kids In Love Whose Families Hate Each Other motif to get the reader to abstractly fill in the blanks as they wish.
That Keats writes his setting as being this weird hodge-podge of literary tropes heightens the unreal feeling of the poem. You feel like, even if you somehow transported yourself back to an actual medieval castle, it wouldn't be this castle, because this one couldn't have ever existed. It's just a random collection of genre conventions. Even as the setting definitely feels Gothic—what with the drafty castles, twisting hallways, medieval feasts, and ancient bloodlust—it also kind of feels like a parody of the Gothic genre.