Craft of building in stone, brick, or block. By 4000 BC, Egypt had developed an elaborate cut-stone technique. In Crete, Italy, and Greece, cyclopean work overcame material weaknesses by using enormous irregularly shaped stones without mortar, thereby reducing the number of joints. African stonemasons also were skilled at mortarless work, and Japanese mortarless castle walls resisted collapse during earthquakes. The Roman inventions of concrete and mortar permitted the development of the arch into one of the basic construction forms and gave rise to a number of variations in the facing used for walls: squared stone blocks, concrete studded with rough stones, concrete with diagonal stone courses, brick- and tile-faced concrete, and mixed brick and stone. The Assyrian and Persian empires, which lacked stone outcroppings, used sun-dried clay bricks. Stone and clay were the primary masonry materials in the Middle Ages and later. Precast-concrete blocks, often used as infill in modern steel framing, did not effectively compete with brick until the 20th century. Brick and block are often combined or used in cavity walls. Glass-block walls, which utilize steel rods to reinforce the mortar joints, admit light and afford greater protection against intruders and vandals than ordinary glass. See alsoadobe, building stone.