The effect of parallelism in this excerpt is that it emphasizes the sizable burden imposed by the summons.
The narrator of the story was invited to the House of Usher, without knowing why. So, obviously, when he uses parallelisms while repeating the word upon to show what he was looking at, he wants to show how troubled he is by this invitation. As it turns out, it wasn't for nothing - there was a serious reason that he should be afraid.
