Thunder in the Dawn by Henry Kuttner

When the Druid priest Dalan tells Elak that the Northmen have invaded Cyrena and King Orander, Elak's brother, is now a captive, Elak knows his duty as a prince of Cyrena. Abandoning his life as a wandering adventurer of Atlantis, he sets off with his companions for Dalan's ship. For only he can rally his people, raise an army, and rescue his brother!