2600 BC marks the Mature Harappan Phase during which Early Harappan communities turned into large urban centres including Harappa, Dholavira, Mohenjo-Daro, Lothal, Rupar, and Rakhigarhi, and more than 1,000 towns and villages, often of relatively small size. Mature Harappans evolved new techniques in metallurgy and produced copper, bronze, lead, and tin and displayed advanced levels of engineering. As seen in Harappa, Mohenjo-daro and the recently partially excavated Rakhigarhi, this urban plan included the world's first known urban sanitation systems: see hydraulic engineering of the Indus Valley Civilisation. Within the city, individual homes or groups of homes obtained water from wells. From a room that appears to have been set aside for bathing, waste water was directed to covered drains, which lined the major streets. Houses opened only to inner courtyards and smaller lanes. The house-building in some villages in the region still resembles in some respects the house-building of the Harappans. The advanced architecture of the Harappans is shown by their impressive dockyards, granaries, warehouses, brick platforms, and protective walls. The massive walls of Indus cities most likely protected the Harappans from floods and may have dissuaded military conflicts.