The narrator describes the mansion as a face with "vacant eye-like windows" and a "gloom" about it. What, other than the house, is he describing?

The most obvious answer could be the occupant of said house: Roderick Usher

Usher is a man plagued by many demons. His face is  "cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison" and his entire being is one of melancholy mixed with anxiety.

The less obvious answer but a good fit all the same, is Roderick's sister Madeline. She is prone to narcolepsy and falling into a death-like state. It is not much of a strech to believe that the Narrator could be desribing her vacant eyes and the gloomness about her being.

All of which sets up the ending, which is detailed in the link below. Hope this helps.