By 1300, a Marcher region had been created between England and Wales, consisting of about forty castle-centered lordships that extended along the Anglo-Welsh border and much of southern Wales. Expressions like “the Welsh marches” are still part of today’s vernacular, though they refer only vaguely to Anglo-Welsh borders—but the question remains: what was this medieval March of Wales, and how and why was it created? This book provides a readerly, scholarly, yet concise answer, aided by maps, illustrations, a list of key dates, and primary source material—placing the March in the context of current debates on frontiers and the medieval British Isles.