Tir Tanagiri, book 1: The King's Peace
Jo Walton2000

Sulien ap Gwien was seventeen when the Jarnish raiders came. Had she been armed when they found her, she could have taken them all. As it was, it took six of them to subdue her. She will never forgive them.

Thus begins her story - a story that will take her to Caer Tanaga, where King Urdo struggles to bind together the squabbling nobles and petty princes into a unified force that will drive out the barbarians and restore the King's Peace