Feebly she laugheth in the languid moon, While Porphyro upon her face doth look, Like puzzled urchin on an aged crone Who keepeth clos'd a wond'rous riddle-book, As spectacled she sits in chimney nook. But soon his eyes grew brilliant, when she told His lady's purpose; and he scarce could brook Tears, at the thought of those enchantments cold, And Madeline asleep in lap of legends old.
We love thinking of Porphyro looking like a puzzled sea-urchin, but here the speaker means "urchin" in the sense of a bedraggled little kid who just can't seem to understand what Angela's getting at.
He hasn't completely caught on to what Angela's saying yet, but he gets that it has something to do with Madeline and St. Agnes' Eve, which may be why he's staring at her as if she's got the keys to a whole bunch of Madeline-related secrets—which, of course, she does.
Porphyro's basically on Angela's side with regards to the ritual. He doesn't think anything's going to happen, and it bothers him to think of Madeline, a true believer, putting her faith in "enchantments cold."
The "legends old" probably refer to the tale of the original Agnes—the long-dead Christian martyr whose story is, of course, the basis for the entire ritual that Madeline's counting on. So, on one hand, our ritual is made out to be an "enchantment," but on the other, it's a dusty historical "legend."