From the time the Romans first set foot on England's shore in 55 B.C., the British Isles have faced a constant threat of foreign invasion. As a result, the landscapes of England, Scotland, and Ireland are dotted with ancient defensive fortifications as varied as their makers. Iron Age Celtic "hill forts," Roman castra and Hadrian's Wall, Anglo-Saxon dykes and Alfredian burhs, Norman mottes and stone-keeps, Edwardian castles, Irish tower houses--they...