Symbol Analysis
If you wanted to get simple, you could describe a lot of this poem's plot in terms of looking: Madeline wants to see Porphyro, Porphyro wants to see Madeline (but doesn't want anybody—including Madeline—to see him). As well, Angela's surprised to see Porphyro and, when she finds out what his plan is, looks at him as though he's a totally different person… we could go on and on.
Looking—and, on the flip-side, being the thing looked at—is a big deal in "The Eve of St. Agnes," especially when you bring into the mix the concept of envisioning something, like how Madeline envisions Porphyro in her dream but isn't necessarily looking at the guy in real life.
We start out with Madeline, who's so focused on getting her magical vision of Porphyro that, in real time, she can't visually process the real world around her. We're introduced to her as she's dancing at the party "with vague, regardless eyes" (64), anticipating when she can get back to bed to enact the ritual—one of the rules of which is that she's literally not allowed to look around her. She can only look upward and wait for this vision to come to her.
Porphyro, on the other hand, creeps his way in the castle, totally avoiding the gaze of any of the party-goers, so that he can see Madeline. Now, if Porphyro just wanted to run away with Madeline, it would have been a lot easier on him if he'd just had Angela lead him to Madeline's bedroom, waited for Madeline to arrive, explained his plan to her and then escaped with her. Instead, he hides in a closet specifically so that "he might see her beauty unespied" (166).
He eventually does wake Madeline up, but he first goes through the rigamarole of preparing a feast and playing some music for her—in short, he goes to great lengths to control the exact circumstances of her seeing him.