The Indus Valley Civilization was an early civilization that flourished along the Indus River from 3000 BCE to 1900 BCE. Two major cities, Harappa and Mohenjo-Daro, had standardized brick architecture and urban planning with road grids and sewage systems. The civilization declined around 1900 BCE for unknown reasons. Centuries later, Indo-European speaking Aryans migrated into India around 1500 BCE, establishing Vedic culture and Sanskrit language. They developed the system of castes that stratified Hindu society into priests, warriors, merchants and laborers.