People erected their dwellings within the city's walls out of mud bricks. The walls were several feet thick to keep the heat out. A typical Sumerian city residence had a succession of rooms arranged around a courtyard. Palm leaves were commonly laid atop wooden beams to cover the courtyard. The roof was made of palm branches tied together with grass. They also used wood for buildings, such as for fences or panels inside houses. Wood is easy to get, and useful for many things. It can be burned for fuel, which helps cool off the house during hot weather, while at the same time giving off water vapor that makes the room humid. Or it can be used to build structures such as platforms or shelves. In fact, the earliest cities were mostly made up of wood, because stone wasn't available in large quantities until much later on. Even after people started using stone, they often built their own walls out of dirt and stones, since materials like cement or asphalt weren't known yet. The Babylonians, for example, built their own wall around their city over 10 times! The first cities were probably made up of tents. Then people started building houses from wood or clay. Finally, after some time, people began building houses out of stone. But even though stone is hard, people still used wood for doors and windows because they were easier to come by than true stone materials.