This seems familiar. Too familiar.

Christopher Stasheff wrote a series of SF/fantasy books back in the 80s about a interstellar agent who fetches up on a planet called Graymarye which is a semi, kinda, sorta medieval society.* Pretty much, as I like to say, what would happen if Markland and the SCA had their own world. Or if Renn Faire was alway and everywhere.

The one that I am reading now is the second in the series, King Kobold Revived. Most of the characters speak rather "yeomanly" but Rod Gallowglass, our hero, speaks in very idiomatic late 20th Century English, as does Yorick, a time traveling Neanderthal**, in case the following excerpt sounds a bit peculiar.

...."There's that little matter of the rumor he's supposed to circulate," [Rod] reminded Tuan.
Yorick shrugged. "That you and your army have really come just to oust Mughorck, isn't it? Not to wipe out the local citizenry?"
"Thou hast it aright."
"But you do understand," Yorick pointed out, "that they'll have to fight until they kow Mughorck's been taken, don't you? I mean, if they switched to your side and he won, it could be very embarrassing for them -- not to mention their wives and children."
"Assuredly," Tuan agreed. "Nay, I hope only that, when they know Mughorck is ta'en, they'll not hesitate to lay down their arms."
"I have a notion that most of them will be too busy cheering to think about objecting."
" 'Tis well. Now..." Tuan leaned forward, eyes glittering. "How can we be sure of taking Mughorck?"
"An we wish a quick ending to this battle," Brom explained, "we cannot fight through the whole mass of beastmen to reach him.

" 'The High Cave'?" Tuan frowned. "What is that?"
"Just the highest cave in the cliff-wall. When we first arrived we all camped out in caves, and Eagle took the highest one so he could see the whole picture of what was going on. When the rank and file moved out into huts, he stayed there -- so Mughorck will have to have moved in there, to use the symbol of possession to reinforce his power."
"Well reasoned," Brom rumbled, "but how if thou'rt mistaken?"
Yorick shrugged. "Then we keep looking till we find him. We shouldn't have too much trouble; I very much doubt that he'd be at the front line."

*Amazon's description: Gramarye is a forgotten colony of Earth, an abandoned planet of telepathic outlaws, where elves and witches are real, and the fantasies of the Middle Ages endure.

**Yeah. Don't ask.