The Indus Valley civilization flourished between 2500-1500 BCE along the Indus River valley. Two major cities, Harappa and Mohenjo-Daro, exhibited advanced urban planning with grid layouts, public wells, and drainage systems. The civilization made contributions to math, writing, and plumbing but declined due to floods and climate change. Around 1500 BCE, Indo-Aryan invaders known as Aryans conquered the region and established a rigid caste system that divided people by occupation and social status. Hinduism and Buddhism later emerged as major religions during this period in ancient India's history.