Eric Knight's short story “Strong In The Arms” (1942 or earlier, I've mostly read it in Hungarian translation from the volume ) talks about the legend of a giant guarding Wada's Keep. Let me summarize what we learn from the story.
There's an old castle called Wada's Keep near the village of Polkingthorpe Bridge (in Yorkshire, England). The locals, including Sam Small, seem to consider it a haunted place, and try not to go there during the night. On the occasion that this short story tells about, Black Cawper, the strongest man in the village, walks to the castle one late evening. He forces Sam Small to come with him, so we learn about this story from his viewpoint.
Wada's Keep is guarded by a giant, a man who's two meters tall and very strong. He says that he is bound to guard the castle from “the invader”. This man appears only at night, probably only for one full moon night every month.
Black Cawper meets the giant. He is curious about meeting such a giant, and so can't just leave. They spends the rest of the night competing him in every manner of sporty activity, comparing their speed and dexterity and strength, all without any animosity. The giant wins every event, just barely besting Cawper in wrestling at the end.
The giant is sad that he didn't lose. Apparently he will be freed from his doom of guarding the tower if he finds someone stronger than him.
Black Cawper died in a mining accident the very day after, performing one last heroic act of strength to save seven miners.
What are the sources for the legend of the giant guarding Wada's Keep? Which parts are original inventions of Eric Knight? What elements did he take from existing folklore?
I believe that Eric Knight got most of the topics for his writing from his own life, so he may have heard some of the legend as oral folklore. But even so, there can be other writings that document this legend and share a common origin, even if they're not writings that Knight himself has read.
: Eric Knight, Sam Small csodálatos élete, (1983, cop. 1979) Móra könyvkiadó, második kiadás, trans. Szerb Antal, illustrated by Hegedüs István, ISBN: 963-11-3283-8, based on (Eric Knight, “Sam Small flies again”, Phoenix publishing co. ltd., Scherz and Hallwag, Berne – Paris, copyright 1937, 1938, 1939, 1940, 1941, 1941 by Eric Knight, copyright renewed © 1965 by Jere Knight)