Another neat image of the past. For the speaker, time isn't just a single straight line, but a kind of mysterious set of corridors. We imagine a labyrinth, the kind of thing you'd see in a Tim Burton movie, full of creepy effects and strange creatures. Combine that with the footsteps echoing down these corridors, and you've got the ingredients for a scary movie. We're not saying Longfellow is trying to scare you exactly; it's just that this seems like an unusually vivid and spine-tingling image in the middle of a pretty quiet poem.
Line 20: In general terms, we'd call this a metaphor. Time isn't a place, it doesn't have any physical existence. When Longfellow says that time has corridors, he's creating an image in our minds, turning an idea into reality. We think that works pretty well here. It helps us to see how important time and history is in this poem, and helps it to really stick in our heads.