The setting is post-Norman Conquest in 1081. Wales is divided into a number of Kingdoms, frequently at war with each other and the Normans based in the border marches.
The focus of the book is an attempt by Gruffydd, in alliance with Rhys ap Tewdwr, to recover the throne of Gwynedd held by Trahern. With the aid of largely mercenary forces hired in Ireland he defeats Trahern and two armies allied with him, Caradog ap Gruffydd of Gwent (South East Wales) and Meilyr ap Rhiwallon of Powys, (Mid Wales) at the battle of Mynydd Carn. The Welsh alliance is funded by the Normans.
There are several sub-plots, with a range of strong characters. Strong battle scenes, treachery etc. All you would expect from good historical fiction. And an unexpected twist at the end that I won't spoil.
This is a period I knew little about and offers an alternative set of scenarios for Norman armies. I will read more in this series.