This novel is a wonderful romp. In 1345, Sir Roger Baron de Tourneville repels blue-skinned aliens who land their ship in his pasture. He boards his forces and the entire village of Ansby on the ship and demands a captured alien take them to France to join the English king. Instead the alien takes them to his planet where he's sure the Earthmen will be easily disposed of. Except... Well, you'll have to read.
The novel is great fun--a short, fast-paced read told as the first person account of Brother Parvus, a monk from the town's abbey. It's a witty sword and science adventure tale with tongue firmly in cheek, likable well-drawn characters and touches of romance in a smooth, light-hearted style.
Anderson may have "better" novels, certainly more serious ones, but I think none that are more fun. Anderson is one of my favorite authors in both science fiction and fantasy. If you're interested in his fantasy, I loved both The Broken Sword and Three Hearts and Three Lions--high fantasy works that don't read like Tolkien retreads. Among his science fiction novels I've read and loved by him are Harvest of Stars and its sequel, The Stars Are Also Fire.