Where It All Goes Down
"The Tide Rises, the Tide Falls" is a very dark poem, literally and thematically. The poem takes place at twilight, near the seashore. Nearby is a town, where a traveler is heading at the end of the day (and at the end of his life). One can almost imagine that the speaker of the poem is watching the whole scene unfold from some higher vantage point nearby, one close enough to the sea and the town that he can hear the sounds of both quite easily.
If the literal setting of this poem is the seashore at twilight, the metaphorical or figurative setting is death, the end of life, the twilight of existence. All the eerie references to darkness and twilight and disappearances (remember those footprints?) make it abundantly clear that the whole scene is supposed to be a picture of death itself. If you could take the whole idea of death and dying and convert it into a landscape, you'd have something kind of like what's described in "The Tide Rises, the Tide Falls."
Part of the reason this poem is set in a very deathly place is because, well, Longfellow wrote it in 1879, three years before his own death in 1882. The twilight described in this poem is firstly a literal twilight, secondly a twilight that symbolizes death itself, and thirdly a figure for the twilight of Longfellow's own life. His tide was definitely falling, and he knew it. Many of the poems in Ultima Thule, Longfellow's last volume of poetry and the one in which "The Tide Rises, the Tide Falls" first appeared, reflect on the approach of death (like this one).