Once a fair and stately palace (3)
The "once" that starts this line is the first hint we get about when this poem is taking place. The first couple of lines give us a really vivid sense of where we are, but then this line puts some distance between us and those descriptions. Suddenly we're in fairy tale time, in the land of "Once upon a time." We think this line actually has a really powerful effect in setting the tone for the poem.
(This—all this—was in the oldenTime long ago,) (11-12)
The speaker is so focused on the "long ago" aspect of his descriptions that he breaks into the flow of the poem to remind us that he's talking about the past. It's almost like he doesn't want us to get too comfortable with the beautiful images he's laying out in front of us. In a way, this foreshadows the eventual collapse of beauty and joy that comes at the end of the poem.
Is but a dim-remembered story (39)
Now we're in the dark part of the poem. The beauty that we saw before has vanished. It's just a dim memory, now that the palace has fallen into chaos and despair. Even though the images of the happy palace are fresh in our minds, the speaker wants us to know that, in the world of the poem, they're hazy, almost gone. Memory and the past become almost ghostlike in this line. Instead of being comforting and uplifting, that happy story of the past is dead and gone, slowly slipping away. (We know, it's a drag, but it turns out that Poe's not super-into happy endings.)