The sixth and final book. Spoilers ahead for the earlier ones.
Mari and Alain, with their forces, head to Dorcastle, as she said at the end of the last book. Alain still anxious about his vision of Mari, dying.
On the way, they discover a nasty machination of the Mages Guild, and get some additional equipment. And arrive for the siege.
Which involves the difficulties of using their far-speakers at that place, lightning-wielding mages on rocs, explosive carts, an elder telling Alain something and another Mage realizing exactly what it meant, assassins who are not quite so secret as they thought after their infiltration, a cousin of Alain's, and more.