The climax of any piece is the moment when things change completely and the action takes a shift, it also should be the most exciting and dramatic moment.
In this poem, the lines that form the climax are: "The mirror crack'd from side to side; / 'The curse is come upon me,' cried / The Lady of Shalott. "
This is the moment when the lady looks out the window to see Lancelot and brings the curse upon herself. This is when everything starts going downhill, and she eventually dies.
