Where It All Goes Down
The main setting of the poem is a trial, but whether the trial is taking place in a courtroom, a throne room, a palace, or outdoors, is entirely up to our imagination. The narrator never tells us explicitly where they are.
The defendant of the trial is Guenevere, the queen of King Arthur, who has been accused (before the poem starts) of cheating on her husband with Sir Launcelot. She is given a chance to defend herself before the court, and that's where the poem begins.
Guenevere's defense is a series of memories that she describes for the court in vivid detail. You could imagine the setting of the poem to shift with each of the memories she recalls: the path to the sea (lines 90-108), the beautiful garden (109-141), the trial by combat (184-221), or Guenevere's own bedroom (241-276). The whole defense takes on a dreamlike quality as the images shift and change.