Why did one wall remain after the fire?
In "The Black Cat," after the narrator cuts an eye out of the black cat that follows him around then hangs him in the garden, his house catches fire and burns down--except for the wall directly behind his bed. He assumes that this is because it had been "recently spread." His efforts to explain the odd phenomena go well beyond this, though. On the wall is a clear "figure of a gigantic cat" with a rope around its neck, which he explains away by recalling that the cat "had been hung in a garden adjacent to the house," so when the fire began, someone in the crowd must have thrown the cat through the window to wake him up; further, when the other walls fell, they must have squashed the cat into the freshly-spread plaster, "the lime of which, with the flames, and the ammonia from the carcass, had then accomplished the portraiture as I saw it."
Of course, neither explanation really holds water. Remember that this is a fictional world; what he's denying is that the supernatural is truly at work here.