Charlemagne's Paladins is an AD&D campaign sourcebook with the radical, yet appealing premise of playing an AD&D game, but set in a fantasy version of medieval Europe.
Oh, okay, that was needlessly catty. This book is really quite good, but I'm still annoyed by all the times AD&D pigeonholed Asian-inspired fantasy as belonging to an "exotic oriental setting." According to this book, in Carolingian France, the job of Parish Priest was so unattractive that the local lords would often have to conscript their serfs into doing it. "Occasionally, the village idiot or another equally useless person would be chosen since he had no other practical value to the landlord."
Damn. But more than being an uncalled-for burn on Christianity, it's such an interesting and specific bit of world-building. I've been playing and running this game for decades and it never occurred to me to answer the question "how was this priest called to service" with "their landlord thought they were useless, so gave them this position in the near-certain conviction that they'd be so weak-willed and ineffectual that the noble lord would be able to come by and raid the tithing box whenever they felt like it."
Could you even imagine if that shit showed up in Greyhawk? Like the priests of Pelor are constantly butting heads with a secularized aristocracy that follows the faith for reasons of cultural identification, but which alternately resents and condescends to its moral teachings for seeming otherworldly and better suited to a docile peasantry. We'd never hear the end of how "sophisticated and complex" OD&D's politics were.
I guess that's the advantage of drawing from real life. Like Age of Heroes, this book has a near-perfect level of detail for rpg use, to a degree that it seems almost shameful that fictional settings don't follow its example. Also like Age of Heroes, the only part of the book that doesn't work is when it tries to apply the AD&D rules.
In theory, there are three different genre settings for your Charlemagne's Paladins game - historical, full fantasy, and the compromise option, "legendary." But oddly enough, you can play a cleric, complete with magical spells, in all three options. In a Legendary game, you can also play a wizard, but only a specialist.
I bring it up, because it's a great example of the game not quite understanding that it can change the rules to suit its setting (or genre, for that matter). Priests (presumably above the level of the local parish, at least) play an important part in the setting, so you can never ban the Cleric class. Because Clerics are priests, and thus priests must be clerics. And you just have to not think too hard about how their spells are a poor fit for Christian miracles.
But with all the magic using classes, the book sort of half-asses their mechanics in order to kind-of/sort-of bring them in line with the setting. Clerics get access to only a limited number of spheres. But spell curation within the spheres is extremely limited. In the sample adventure, one of the antagonists uses Darkness and Call Lightning, despite the fact that it was ostensibly for the "historical" version of the setting. It's sloppy.
But if you accept that you pretty much have to build all the rules stuff from the ground up (or, better yet, use this setting to play an all-Martial game of 4th edition), then Charlemagne's Paladins is an admirable setting book. In fact, there are significant swaths of this book that I wish would just get appended to any fictional setting that proclaimed itself "standard medieval fantasy." If it then turned out that doing so would too dramatically change the nature of the world, then maybe the authors could rethink a thing or two.
Ukss Contribution: I'm going to go with a detail that charmed the hell out of me because it was exactly the sort of weird thing that real medieval Europeans were always doing, but which never winds up making it into "medieval fantasy." In one of the romances to spin off from the Song of Roland, the warrior Ogier the Dane performed a great deed of heroism and was rewarded by Charlemagne with a blade that bore the inscription, "Wear Me Until You Find A Better."
It's good to remember that the ancients also liked to have fun.