Stol'n to this paradise, and so entranced, Porphyro gazed upon her empty dress, And listen'd to her breathing, if it chanced To wake into a slumberous tenderness; Which when he heard, that minute did he bless, And breath'd himself: then from the closet crept, Noiseless as fear in a wide wilderness, And over the hush'd carpet, silent, stept, And 'tween the curtains peep'd, where, lo!—how fast she slept.
Madeline's got one of those fancy canopy beds with curtains that everybody wanted as a kid (and which we, as adults, still want). Anyway, the point is that Porphyro can't see her at the moment, so instead he looks at her dress, which, as you remember from stanza 26, is Keats's way of outlining Madeline's body without actually talking about her body.
In the last stanza, we saw that Madeline has entered this weird "wakeful swoon" state, and here Keats hints that she's not alone there: Porphyro is also inside that otherworldly "paradise," even though he's on the other side of the room, stuck in a closet.
Deciding that he's stared at the dress long enough, Porphyro decides he's going in and, emerging from the closet, approaches the bed.
Porphyro's approach across the room is "noiseless as fear in a wide wilderness," which is a funny simile because it a) characterizes Madeline's bedroom as a "wilderness" and b) likens Porphyro to "fear." He's been sneaking around this castle full of "barbarian hordes" (85) all night, but now, when he's in the safest, most domestic place, that place is wild, and he himself is "fear."
What do you think of the fact that this scene is made out both to be a "wilderness" and a "paradise?"