Prisoners. The Fire Tree Series, #8

The Prisoners is the eighth book in The Fire Tree, a sweeping historical fantasy series set in the Scottish Highlands of the early 1600's. Consigned to Woden's Well - a deep, circular stone pit in the bowels of The Fortress of Inverness - Queen Annis and her rival Janine face a darkness far greater than the absence of light. Colonel Drew Jamieson, a sadistic officer serving the Duke of Cumberland, intends to murder them both and plunge Scotland into civil war.
The chaos will open the door for a French invasion and allow a cabal of English conspirators to carve up the Highlands between them. Their captor has made a critical miscalculation. In the depths of their prison, stripped of everything but each other, Annis and Janine uncover a secret that has been buried since infancy: they are twin sisters, both daughters of the murdered Queen Cydara. The rivalry that defined their lives was a deception born of a mother's desperate love.
The crown they fought over belongs to them both. Above ground, Constable Ewan Burberry languishes in chains while ancient prophecies come alive around him. The Duke of Bo'Ness hatches a plan so audacious it depends on recruiting pirates. Hamish Pottle and Neacal O'Keefe escape through a labyrinth of ingenious secret tunnels and traverse the Highlands under the protection of a disguised MacDonald army.
At Burghead Bay, they strike a devil's bargain with a pirate chief who owes the Queen of the West a debt of honour. Meanwhile, Captain MacOwen plays the most dangerous game of all - feigning loyalty to a man he despises while waiting for the moment to turn. Two young twins - The Spiders - crawl through the chimneys. The Brydda stir from eight centuries of silence. Wild Flower, the ageless seer, moves between the scattered allies like a thread stitching together the fabric of fate.
Loyalties are tested, sacrifices are made, and the "Englishmen's Field" is consecrated in blood and fire. As the Quickening burns brighter, the time for heroes draws near. The Prisoners is a tale of sisterhood, sacrifice and the unbreakable bonds that hold a kingdom together when all seems lost.