William built hundreds of motte and bailey castles in the i rst few years he ruled England. Once he was satisi ed that his control of the country was secure, he took the next crucial step in castle evolution—rebuilding those earthen and wooden fortii cations, only this time using stronger and more permanent stone. Rulers and members of the nobility, not only in the British Isles but also across continental Europe, rapidly followed suit, erecting their own stone castles. Imposing and dii cult to capture, stone castles transformed medieval warfare. In fact, sieges of castles became the most common form of military action during the remainder of the medieval era. Furthermore, the evolution of siege weapons and tactics produced a relentless arms race—each new of ensive advance was matched by a defensive one, and vice versa. Only when ef ective cannons appeared in the i nal medieval years were stone castles rendered obsolete, and this development was one of the principal factors marking the transition from medieval to modern warfare. All of these critical developments were the far-reaching legacy of a single, strong, and innovative military leader who boldly led an army across the English Channel and into the history books.