This is the first book in the Chronicles of Tornor by Elizabeth A. Lynn. Fantasy where the Warriors of death meet the Warriors of dance.
The story of Ryke, a soldier of Tornor Keep, a harsh castle in a wintry land. When the fortress is taken by raiders, Ryke stays loyal to his prince (now lord, since the old lord was killed in the battle), and contrives a way for them to escape the fort with the help of two neutral messengers, Sorren and Norres, whose gender and identities are secret....
They travel to a secret valley, named after a mythical land of always-summer, where an exiled soldier, Van, is teaching small groups of followers a martial art that is somewhere between judo and capoeira, infused with a new spiritual philosophy of balance. They set out to re-take the keep - but harsh lessons are in store before each of the characters learns that their path in life may not be what had always been expected of them.
It's a short book, but for a first novel, excellent. Still, I feel if Lynn had written this later in her career, she probably would have fleshed out both the characters and the complex details of their world to a greater extent.