The inner walls, which glowed white with a protective layer of limewash, were made of sun-dried mud bricks. This mixture of clay, sand and straw was layered above stones that insulated them from moisture. Dr Susannah Hagan, emeritus professor of architecture at the University of Westminster and an expert on green architecture, later explained why this building technique is so ingenious: "The secret is in the walls: thick walls of earth or stone that delay the sun"s heat penetrating a building interior during the day, and radiate that heat back to the cold sky at night," she said. "By morning, the walls have cooled enough to start the protective cycle again."