The green branch - Edith Pargeter

Young Harry Talvace, the son of Ralf Isambard's master-builder who raised the great church of Parfois and was put to death by his jealous patron, has grown up at the court of Llewelyn, Prince of North Wales.
Deep in his heart he nurses a desire for vengence, and when Harry become innocently involved in the tragedy which strikes Llewelyn's marriage he sets out to avenge his father's death.

Alone he makes his way to Parfois to challenge Isambard.
But enmity can prove as complex as love, Harry discovers, as in his turn he falls under the spell of the old warrior.