The basic premise is that in 1345 an alien spaceship lands in a village in England, intending to intimidate the barbarian locals and conquer the planet. Only the English react to the intimidation by fighting back & defeat the aliens. They then hit upon the idea of forcing the alien they kept alive to use the spaceship to fly them to the Holy Land, where they were preparing to go on Crusade to anyway. Instead the alien betrays them & takes them off to the planet it came from, and they subsequently lose the navigational notes that would let them get home - so they have to survive on this alien world, by a combination of bluff, diplomacy and the war-like nature of the High Medieval society.
The very brief framing story is the rediscovery of the descendants of this village by the descendants of us on earth, and the story proper is the chronicle as written down by a monk who was there when it all happened. And it feels medieval-ish, there's a lot of concern from the monk & other clergy about if the aliens had souls or not, the monk teaches the alien Latin because then he'll be able to talk to anyone educated from anywhere in the civilised world. It's also an entertaining counterpoint to the sub-genre of post-apocalyptic stories where SCA members & re-enactors end up running the world because of their interest in medieval skills - here we have the "real thing" surviving out on alien worlds because of their medieval skills. From the posts at tor.com I gather that Anderson was friends with the initial founders of the SCA and all this was around the same time period as he was writing the book, so it fits in quite nicely :)
Overall, a fun read :) A bit of extra suspension of disbelief required because 60s ideas of alien tech are not very similar to modern ideas (no computers - the lost navigation notes were on paper!), and I think it would be harder to plausibly write the novel nowadays, I'm not sure we'd buy the idea of 14th Century nobles/clergy/peasants working out how to run a modern idea of a hi-tech computer driven spaceship.