In this fascinating poem a lot seems to be said about the division between art and life. The Lady of Shallot is trapped or "embowered" in her tower and left to see life as a pale imitation as mediated through her mirror. She spends her time recoring life in her embroidery, a "magic web with colours gay." We hear about the curse that is on her, that if she were to look down to Camelot herself without using the mirror, her doom would come upon her.
So, quite clearly, the climax of this poem comes when the Lady of Shallot is so overwhelmed by reality that she is tempted into looking down at the sight of Sir Launcelot trotting past her window. Note how he is described with such vibrancy:
A bowshot from her bower eaves,
He rode between the barley sheaves,
The sun came dazzling through the leaves,
And flamed upon the brazen greaves
Of bold Sir Launcelot.
In this poem, we can see from the dramatic way in which he enters, that he could be said to symbolise the forces of life and reality. The impact this sight has on the Lady of Shallot is enough to bring the doom upon her:
She left the web, she left the loom
She made three paces through the room,
She saw the waterlilly bloom,
She saw the helmet and the plume,
She looked down to Camelot.
Note how the repetition of the phrase "She" functions to increase the dramatic nature of this stanza. This is the climax of the poem because it results in the irrevocable breaking of the mirror and her abandonment of her tower.