The same period saw the evolution of the fortified stronghold from the Anglo-Saxon motte-and-bailey, a timber tower encircled by a timber and earth wall, to the formidable, fully developed masonry castle that had become an anachronism by the end of the Middle Ages because of the development of artillery. Intrinsic to this innovation were the invention of gunpowder and the development of techniques for casting metals, especially iron. Gunpowder appeared in western Europe in the mid-13th century, although its formula had been known in East Asia long before that date. It consists of a mixture of carbon, sulfur, and saltpetre, ...(100 of 38754 words)