Simon the Coldheart is one of Georgette Heyer's earliest works, written before she really hit her stride with books set in the Regency period. This story is set much earlier - in the 15th century - and follows hero Simon Beauvallet, a nobody who works his way up from poverty to a knighthood and becomes a friend of the future King Henry V. Because of the date of the story, the language feels more Shakespearean than Heyer's Regencies, and the old-fashioned language might not appeal to all readers (although I personally liked it).
This isn't a medieval romance; it's more a mixture of different elements that make up an enjoyable, if perhaps less accomplished, story. We follow Simon as he works his way up in the world, as he fights battles, and as he eventually finds himself up against a very worthy opponent, Lady Margaret of Belrémy.
Some good scenes break up less effective ones, and aspects of the writing don't entirely ring true. Simon is a quiet, self-possessed man who some think cold of heart (thus his name), yet he clearly knows his mind and has worthwhile things to say when he says them. Simon the Coldheart shows the clash of two cultures after the Battle of Agincourt and gives detail of life in medieval times, but it's so different from Heyer's later Regencies and betrays at times that she was a young author trying to find her voice.