They glide, like phantoms, into the wide hall; Like phantoms to the iron porch they glide; Where lay the Porter, in uneasy sprawl, With a huge empty flagon by his side: The wakeful bloodhound rose, and shook his hide, But his sagacious eye an inmate owns: By one, and one, the bolts full easy slide:— The chains lie silent on the footworn stones;— The key turns, and the door upon its hinges groans.
Porphyro and Madeline are made out to not be human themselves—the simile here describes them as "phantoms" as they cross the final stretch to the big doors.
They can get past the Porter—the gatekeeper—just fine, on account of him being totally passed out. Way to do your job, Porter.
The bloodhound (of course there's a bloodhound, because we have to hit all the medieval castle clichés) recognizes Madeline (she's the "inmate"), but appears to be okay with letting her pass. "Sagacious" here means wise or smart, and you should think of "inmate" not as a prisoner but as a resident in the castle. In other words, he recognizes Madeline from living in the castle, so he doesn't get all barky about seeing her.