The Ringed Castle

Crawford is rescued from Volos by the clever, beautiful and power-hungry Kiaya Khatun and they travel to Moscow, where he is to train the Tsar's army. Determined never to return to Scotland, he summons some of the mercenaries from the force he trained in Scotland and has to negotiate with a volatile and brutal Tsar, in an insular society held back from greatness by the extremes of its climate and the lack of communication with Western Europe. Philippa has taken the child back to Scotland, and now becomes embroiled in the English court of Mary Tudor, for which desperate hopes for an heir, the absence of Mary's Spanish husband, and England's return to the Catholic faith of England are the chief, if concealed, interests. From London, merchants set out to establish a trade route with Russia and their adventures join with whose of Crawford and his companions. Eventually, Crawford is forced by the Tsar's desire for munitions to return to Britain, and the mystery of his parentage, which Philippa has been investigating, deepens.

This is an historical romance and the fifth of 6 books set in the mid 1500s and focused around a flawed hero, Francis Crawford of Lymond. The series starts and ends in Crawford's (and the author's) homeland of Scotland. The books follow Dunnett's hero through a series of adventures at the centres of power in Scotland, France, Malta, Stamboul (Istanbul), and Russia. He develops as a leader in war and politics, with the potential to rule a country: but at the expense of his humanity, his family and his companions, as he ruthlessly suppresses his own weaknesses and frailties.
The language, culture, customs, political intrigue, warcraft and ethos of the time are captured in beautifully constructed prose and the books are worth reading for this alone. But they are also cracking adventures. If you can, ignore the author's constant reminders of her hero's beauty and stick with them.