In the modern mind, the term ‘hostage’ conjures up someone taken by force – the Iranian Embassy hostages; the person grabbed by a gunman in a robbery. But in Empire’s Hostage, Book II of the Empire’s Legacy series, ‘hostage’ is used in an older way.
“What does it mean, to be a hostage?” I asked. I saw something flicker in Turlo’s eyes. He grinned again.
“Exchanging the children of high rank as hostages is an old and honoured tradition,” he answered, “although not one we have respected, in some generations, and in truth needed to be reminded of. We’ll treat Donnalch’s son, and the other boy they are sending—his brother’s son—with every courtesy. They will lodge in the White Fort for now, and then be sent south to the Eastern Fort when the weather improves, to learn with our senior cadets. Darel, you will basically live the life that Donnalch’s son would have, whatever the education, in arms and tactics and books, they deem appropriate. That is the gist of it: we exchange our heirs, in surety for each side’s good behaviour. You will not be mistreated, but, understand, neither will you be truly free.”
In Hostages in the Middle Ages, Adam Kosto points out that:
Who were these hostages? In her review of Kosto’s book, Shavana Haythornthwaite tells us the preference was for sons of the family, but ‘the question of exactly who a hostage was in the Middle Ages was in fact part and parcel of the question of what the structures of power were.’ And that’s who stands as hostage to the treaty in my book.
He grinned. Nothing, ever, seemed to keep Turlo’s spirits down. “But the treaty, my lad, and lassie,” he added, “requires hostages. Donnalch’s son and another to us, and two children of our leaders to them.”
But peace treaties weren’t the only reason for hostages, and the interpretation can be broad:
In later books in the series, almost all these broad definitions of hostage are part of the story, just as they were part of life in the middle ages.
 Kosto, Adam J. Hostages in the Middle Ages, 2012, Oxford University Press: https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199651702.001.0001/acprof-9780199651702
 Medieval Hostageship c.700-c.1500: Hostage, Captive, Prisoner of War, Guarantee, Peacemaker. Matthew Bennett & Katherine Weikert, eds., Routledge, 2019