The Shattered King. The Gareth & Gwen Medieval Mysteries, #17

Gareth & Gwen travel deep into enemy territory in the seventeenth Gareth & Gwen Medieval Mystery!  January 1151. For most of the past year, Gareth and Gwen have been trapped between armies. Although once allies, King Owain of Gwynedd and King Cadell of Deheubarth are now at odds, to the point that Cadell has invaded Ceredigion, seeking to recapture lands he claims Owain stole in a past war. In so doing, he has set his half-brothers against their own cousin.
Although Gareth and Gwen are holding out hope that more bloodshed can be averted, by January of 1151, Cadell has laid siege to Prince Hywel's castle at Aberystwyth. And then the war takes an unexpected turn for everyone involved .Complete Series reading order: The Good Knight, The Uninvited Guest, The Fourth Horseman, The Fallen Princess, The Unlikely Spy, The Lost Brother, The Renegade Merchant, The Unexpected Ally, The Worthy Soldier, The Favored Son, The Viking Prince, The Irish Bride, The Prince's Man, The Faithless Fool, The Honorable Traitor, The Admirable Physician, The Shattered King.
Also The Bard's Daughter (prequel novella).