The Cry Of The Icemark by Stuart Hill.

Thirrin Freer Strong-in-the-Arm Lindenshield is barely 14 years old when her father the king dies heroically defending his small northerly kingdom. When the invaders come back with the largest army the world has ever seen young Queen Thirrin must assemble her own army of unlikely allies including vampires, werewolves and other creatures of legend in a desperate attempt to hold them off.

Thoroughly enjoyable fantasy yarn. First of a series apparently but the story is entirely self contained.