This question of course goes to the very heart of the theme of the poem. Many critics regard this poem as being about the process of artistic imagination and how we can achieve incredible things through that artistic imagination, in the same way that Kubla Khan imposes order on nature to create the "pleasure dome." The description of the pleasure dome with its beauty and its wonder certainly support the idea that this poem celebrates the imagination and what it can achieve, but at the same time, there is a darker note introduced into the poem by reference to the "tumult" that exists outside the "pleasure dome" that the imagination has created. Note how this tumult represents a shift in imagery and tone, as the imagery becomes more frightening and the tone becomes more sinister:
A savage place! as holy and enchanted
As e'er beneath a waning moon was haunted
By woman wailing for her demon-lover!
The fact that from the "tumult" that exists outside the "pleasure dome" he has created come "ancient voices prophesying war" suggests that whilst the powers of the imagination are incredible and capable of creating things of great beauty, at the same time there is a "dark side" to the imagination that must not go unheeded.