Symbol Analysis
Things get crazy when Madeline opens her eyes, though:
Her eyes were open, but she still beheld,
Now wide awake, the vision of her sleep[…]
While still her gaze on Porphyro would keep;
Who knelt, with joined hands and piteous eye,
Fearing to move or speak, she look'd so dreamingly. (298-306)
First of all, it's unclear what Madeline's seeing and whether or not she's actually seeing it or just imagining it, still stuck in her dream. Whatever the truth, all of the seeing power is immediately and totally transferred to Madeline. She's the one "beholding" the kneeling, powerless Porphyro, who's suddenly so freaked out that he can't move. He himself isn't written here as looking at all; his eyes only serve to characterize him as "piteous" in the gaze of Madeline, which is locked squarely on him. Now that Madeline's the one looking and Porphyro's the object, the power dynamic is totally reversed, and he's forced down the floor—still, pale, and cold. Yeah, how you like that, creepy-sneaky-sorta-boyfriend?
For the rest of the poem, it's never totally clear what Madeline's seeing, or even what we're seeing as readers. For instance, when Madeline and Porphyro finally break out of the castle, Madeline's "beset with fears, / For there were sleeping dragons all around" (352-353). We don't know if Keats is just being dramatic, if there are actual dragons (it's not a huge stretch—ever tried counting how many mentions of fairies we get in this poem?), if Madeline sees her sleeping relatives and merely imagines them to be dragons, or even if she imagines them and that somehow creates them as dragons in the real world.
Or, you know, it could be… all of those at once. Welcome to the world of Romantic poetry. Enjoy your stay.
Madeline, in particular, is depicted as capable of seizing upon an object with her gaze and recreating it with her imagination, and her ability to physically look at something is frequently crosswired with the ideas of creativity and imagination.