The Indus Valley Civilization (c. 3300–1300 BCE) stands out for its remarkably advanced town planning. Cities like Mohenjo-daro and Harappa followed a grid pattern, with streets intersecting at right angles, and were divided into a raised Citadel (housing public buildings) and a Lower Town (for residences). Baked bricks followed a standardized 1:2:4 ratio across sites, while a covered drainage system connected homes to street sewers, reflecting an unusual civic emphasis on sanitation. The Great Bath at Mohenjo-daro suggests ritual bathing practices, and large granaries point to organized food storage. The civilization's decline, beginning around 1900 BCE, was gradual and multi-causal rather than sudden. Key factors include weakening monsoons and rising aridity, the drying up or shifting of the Ghaggar-Hakra river system, and a decline in trade with Mesopotamia. The once-popular Aryan Invasion Theory is now largely discredited due to lack of supporting evidence. Modern scholarship favors a process of de-urbanization and regionalization, with Late Harappan communities shifting to smaller rural settlements rather than vanishing entirely.