Alliteration is an extremely important part of "Fern Hill." Not only does it add to the poem's musicality, but it creates an almost chant-like sound, leaving the reader "spellbound" by the poem's extravagant music.
The second stanza contains some of the poem's most intense alliteration:
Golden in the mercy of his means,
And green and golden I was huntsman and herdsman, the calves
Sang to my horn, the foxes on the hills barked clear and cold,
And the sabbath rang slowly
In the pebbles of the holy streams.
In these lines, the speaker first expresses the joy of feeling like all the animals respond to the speaker. The phrase, "And green and golden I was huntsman and herdsman," with its echoing /h/ and /g/ sounds, captures the speaker's excitement. Then, the speaker describes how the landscape itself feels holy. The lines shorten and the alliteration begins to slow down, mimicking how "the sabbath rang slowly / In the pebbles of the holy stream." The delay of the last /s/ sound suggests time slowing down to a standstill, a sense of peace at the end of the stanza.
A similar effect occurs in the fourth stanza:
So it must have been after the birth of the simple light
In the first, spinning place, the spellbound horses walking warm
Out of the whinnying green stable
Here, the speaker imagines the beginning of the world, according to how the Bible tells it. This is a truly rapturous event, where light, the earth, and animals are all created. After being created, the horses are "spellbound," mesmerized by what has happened, and the alliteration captures this amazement. The hushed sibilance of those /s/ sounds works with the quiet /w/ sounds to create a feeling of wonder and reverence, while the stronger /b/ and /p/ sounds add yet more interest and excitement to the lines.
At the end of the poem, the rapture of alliteration has turned into a much sadder sound. For instance, in the last stanza, the phrase "the farm forever fled from the childless land" conveys the sudden sadness at realizing that childhood is over forever. The repeated /f/ sounds suggest that the speaker speaks this line emphatically and sorrowfully, spitting out these words in sadness and frustration.
The final lines, "Though I sang in my chains like the sea," have an even more sorrowful, wistful air to them. Separated by a bit more space than other occurrences of alliteration (such as the previous example), these /s/ sounds again convey a sort of slowing down. This time, however, they don't convey any sort of peace, but rather an experience of intense melancholy, even suffering.