"The Murders in the Rue Morgue" is only thirty-odd pages long, but it packs in a lot of detail. As we get to in our "In a Nutshell" section, the point of this story is to give the reader a laundry list of clues so that we can follow along with our detective's thought processes. It's like a thousand-piece jigsaw puzzle in which we have to assume that only half the pieces fit together and the other half can be thrown away – but which half? And sure, Dupin, the protagonist, comes out at the end to show us how everything works, but we still have to look at each piece individually before we get that far.
So there's a lot to keep track of, and not all of it is in terms that contemporary readers necessarily get – the Goddess Laverna, Stereotomy, even the Ourang-Outang are all kind of tough to figure out. So we find this story pretty challenging, but remember: it's supposed to be. It's not just that Dupin is trying to solve a mystery. It's a game for the reader, too, and Poe is intentionally giving us more information than we need to make it harder and, therefore, more fun.