"Up-Hill" doesn't have much in the way of elaborate poetic imagery, but the scarcity of such images makes the ones it does have stand out all the more. In this case, we're speaking (literally) about the inn at the top of the hill—you know, the one you "cannot miss." No matter which interpretation of the poem you use, the inn always represents a destination, a comfortable place, and a source of safety at the end of a hard day's journey.
Lines 5-6: The first time the inn is mentioned, we don't actually know what kind of building Rossetti is talking about. The fact that she singles out the roof, however, emphasizes that the primary role of the inn is to provide shelter and safety when night begins to fall.
Lines 7-8: The inn as an inn: By clarifying that the roof in line 6 actually belongs to an inn, Rossetti adds a level of comfort and ease to the situation. This place is not only impossible to miss, it's also prepared and designed to provide rest and comfort to weary travelers.
Lines 7-8: The inn as death: If the journey up the hill is life, then the inn at the top of the hill is left to represent death. It's not so bad as it sounds, though. Death is portrayed as a safe haven, a respite from the hardships of the world.
Lines 7-8: The inn as heaven: Perhaps one reason the inn is portrayed in such a favorable light is because, in a Christian interpretation of the poem, it represents heaven. Declaring that the inn cannot be missed in spite of the darkness is another way of describing how Christians will be saved from the eternal fires and damnation that awaits sinners in hell.
Lines 11-12: The security of the inn is further reinforced by these lines. Not only is the inn waiting at the top of the hill, it's prepared for Speaker #1's arrival.
Lines 15-16: This, folks, is the crowning jewel as far as Biblical interpretations go. Not only is this verse an echo of John 14:2, which describes heaven as "a house with many rooms," it's also a shout out to our pals Mary, Joseph, Baby Jesus, and the inn in Bethlehem. Unlike Mary and Joseph, who were told there was no room for them in the inn, "Up-Hill" promises that, thanks to Christ, there will be room in heaven for everyone who seeks salvation.