Final answer:
Without the context from the source material about the Green Knight at Camelot, an accurate completion of the sentence is not possible. The provided texts describe different knights and their encounters, with detailed depictions of chivalry, romance, and legend.
Step-by-step explanation:
In the texts you've provided, they describe several knights and their dramatic appearances, each with unique characteristics and storylines. However, none of the excerpts you provided specifically mention the Green Knight's dramatic appearance atop any particular thing at Camelot. Therefore, without the specific context from the original source material, it is not possible to accurately complete the sentence regarding the Green Knight's appearance at Camelot. The excerpts refer to various knights in literature, such as a red-cross knight, Sir Lancelot, the Masked Rider, and characters like Don Quixote, who mistakes windmills for giants. The most specific mention to Camelot refers to Sir Lancelot, who is described as dramatic and admired by the Lady of Shalott.