The principal streets of the Harappan cities were laid out in a grid layout. They were constructed from north to south and east to west. The residences constructed on street corners were rounded to allow carts to pass through. Mohenjo-main daro's street was 10.5 meters wide and 800 meters long. Its houses had flat roofs but no walls. There were open spaces between buildings that may have served as courtyards or used for other purposes. The Harappans built their homes with a mixture of mud bricks and stone slabs. Some houses had wooden frames but the majority were only made of mud bricks. The Harappans were also known for their complex drainage systems. Their roads were designed so that any water that did not evaporate would flow into small channels which led to large public tanks where it could be collected for drinking or irrigation purposes. Harappa was primarily a trading center, but also played an important role in agriculture. It is estimated that about 5,000 people lived there at its peak in 2500 B.C. However, due to wars, natural disasters, and poor management decisions, most of the cities fell into disrepair and were abandoned by 500 B.C.