and made it possible to produce tools which were stronger, lighter and cheaper to make than bronze equivalents. The raw materials to make iron, such as ore and limestone, are far more abundant than copper and especially tin ores. Consequently, iron was produced in many areas. It was not possible to mass manufacture steel or pure iron because of the high temperatures required. Furnaces could reach melting temperature but the crucibles and molds needed for melting and casting had not been developed. Steel could be produced by forging bloomery iron to reduce the carbon content in a somewhat controllable way, but steel produced by this method was not homogeneous. In many Eurasian cultures, the Iron Age was the last major step before the development of written language, though again this was not universally the case. In Europe, large hill forts were built either as a refuge in time of war or sometimes as permanent settlements. In some cases, existing forts from the Bronze Age were expanded and enlarged. The pace of land clearance using the more effective iron axes increased, providing more farmland to support the growing population. Mesopotamia Mesopotamia (modern Iraq) and its peoples (Sumerians, Akkadians, Assyrians and Babylonians) lived in cities from c. 4000 BC, and developed a sophisticated architecture in mud-brick and stone, including the use of the true arch. The walls of Babylon were so massive they were quoted as a Wonder of the World. They developed extensive water systems; canals for transport and irrigation in the alluvial south, and catchment systems stretching for tens of kilometers in the hilly north. Their palaces had sophisticated drainage systems. Writing was invented in Mesopotamia, using the cuneiform script. Many records on clay tablets and stone inscriptions have survived. These civilizations were early adopters of bronze technologies which they used for tools, weapons and monumental statuary. By 1200 BC they could cast objects 5 m long in a single piece. Several of the six classic simple machines were invented in Mesopotamia. Mesopotamians have been credited with the invention of the wheel. The wheel and axle mechanism first appeared with the potter's wheel, invented in Mesopotamia (modern Iraq) during the 5th millennium BC. This led to the invention of the wheeled vehicle in Mesopotamia during the early 4th millennium BC. Depictions of wheeled wagons found on clay tablet pictographs at the Eanna district of Uruk are dated between 3700 and