The Fort as River's Bend by Jack Whyte (Camulod Chronicles #5)

Used; Paperback - excellent; 1st edition, some minor shelf wear, a slight curve in the back cover. 

The Fort at River's Bend is the fifth book in Jack Whyte's saga of the creation of King Arthur's Britain

Merlyn Britannicus, leader of the colony known as Camulod, is faced with the task of educating his young charge, Arthur, future King of the Britons. Fearing for the life of his nephew when an assassination attempt is thwarted, Merlyn takes Arthur and his boyhood companions Gwin, Ghilleadh, and Bedwyr, to the ruins of a long-abandoned Roman fort far from Camulod. Once there, Merlyn realizes it's time for Arthur to become worthy of the sword he is destined to wield later in his life-the mighty Excalibur.

But beyond their idyllic hiding place, forces threaten the tenuous peace of Camulod. In Cambria, the death of Arthur's father Uther has left his people leaderless, and in Cornwall, Merlyn's enemy Peter Ironhair is gathering forces to destroy all Merlyn holds dear.

And Merlyn himself is struggling, because in order to make his dream of a united Britain real, he must put the person he loves most in the world in mortal danger-he and Arthur must return to Camulod.