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.