The High Tech Knight by Leo Frankowski (1989, Paperback, Illustrated)

Somehow, Conrad found himself under investigation by the Inquisition, got himself knighted, was granted his own fief, and made a few enemies.

Somehow, he had to round up a few vassals, build himself a city, and figure out how to survive armed combat against the Champion of the Teutonic Knights, one of the Toughest Men Alive.

Then he'd have time to worry about the Mongols . . .