One of the reasons why this poem is so famous and has been such an enduring success is the way in which it uses assonance and alliteration to create haunting sound effects, resulting in a poem which has real verbal music. These sound effects almost lull us as a lullaby would lull a child, helping us to imagine the idyllic setting that is being described to us.
Note how this operates in the following example of the poem:
And I shall have some peace there, for peace comes dropping slow,
Dropping from the veils of the morning to where the cricket sings;
There midnight's all a glimmer, and noon a purple glow,
And evening full of the linnet's wings.
There are lots of examples of alliteration (the repetition of consonnant sounds) and assonance (repetition of vowel sounds) in this stanza. Note the alliteration in "s," "p," "d," "m," and "n." Likewise there is assonance in "slow" and "grow" and "midnight," "linnet," "glimmer" and "wings." These are examples of how Yeats creates the word music that makes this poem so excellent.
Note the way as well in which the last stanza suggests that the speaker feels a mystical connection with nature that endures even in spite of the intrusions of city life. He is always able to hear the "lake water lappping" on the shore of Innisfree, in spite of the "pavements grey" of his environment. This is something he hears in his "deep heart's core," and cannot be ignored or drowned out by the urban sounds that oppress him so.