Castaways in Time # 2: The Seven Magical Jewels of Ireland by Robert Adams

Drawn through a hole in time and space, twentieth-century American Bass Foster finds himself hailed as a noble warrior and chosen to command—first on land and then at sea—the armies fighting to preserve King Arthur III and his realm against the Church-led forces determined to place Arthur’s nephew on the throne of England.

But Bass is not the only traveler in time. And though the mysterious force which has exiled him to this land of knights in shining armor has brought other unexpected comrades-in-arms, it has also opened the gateway to a far future time and place. And suddenly Bass, Arthur, and their allies must face the menace of an unknown but deadly enemy seeking not only to overthrow Arthur’s kingdom but to conquer and enslave their whole world….