What is the significance of the narrator saying that he would ‘build that dome in air`?

Marianne Moore famously stated that poetry was composed of
'imaginary gardens with real toads in them`.
Like Moore`s imaginary garden or Tennyson`s Palace of Art, the pleasure dome of Kubla
Khan is not presented as a real place but as an ideal symbol of artistic
creation.

The key phrase which signals that the dome is
noty a real but an imaginary construct is:

Could I revive
within me
Her symphony and song,
To such a deep delight 'twould win
me,
That with music loud and long,
I would build that dome in
air,

You cannot actually build a dome with music, but only
with concrete building materiasl. Thus the sense is that music (or Coleridge`s poem)
conjures up in the listener`s mind the image of a dome of pleasure, which itself is
merely a symbol of a certain type of ecstatic or Dionysian vision of art and its
associated emotions.