Based on an episode of Bonanza?
No, I'm not kidding. In fact if the episode of Bonanza hadn't come first, I would be positive the episode was based on this movie.
At about the middle of its run, there was an episode of Bonanza where Little Joe is knocked unconscious and wakes up in a very strange, very surreal town inhabited by very strange, very surreal townsfolk. He learns that the townspeople allowed a previous sheriff to be ambushed and killed - and that the killer is going to return soon with more men. Little Joe is subsequently tasked with preparing the town against the killer and his gang. Initially the townspeople are cowardly and reluctant to assist - but eventually they come around and successfully defend the town when the killer and his gang arrive. At the end of the episode, Joe shoots the killer dead after the townspeople have dispatched the gang - only to once again be knocked unconscious and to wake up to find the town deserted and Ben, Hoss, and Adam - who had been searching for him for days - by his side. When Joe relates the story to his family, Ben replies that the entire town has been a ghost town for years - although there is a legend that the town had been cursed since allowing a sheriff to die, and that the curse could only be lifted when an outsider rallies them to defend their town in order to set the misdeed against the sheriff right.
Like High Plains Drifter, the episode is very unusual for the western genre in that there is a very surreal, dreamlike quality throughout. Additionally, it is notable for having a rare supernatural spin hardly ever seen in other episodes throughout the show's 14 year run. Finally, like High Plains Drifter the episode ends on an ambiguous note. Were the townspeople real? Did Joe enter a supernatural state of limbo in order to help lift the curse? Or was it all just a dream he had after being knocked unconscious? Again very unusual for an entry of Bonanza, a series that if anything was somewhat infamous for almost never leaving anything up to the viewer's imagination.
Probably just a coincidence - but who knows? Maybe one of Eastwood's most acclaimed films was actually an expanded version of a TV episode of Bonanza.