The greatest achievement of the Indus Valley were the cities of Harappa and Mohenjo-Daro with their impressive, organized and regular layout. The cities were unique in the sense that they showed urban planning employed in ancient times.
For further reading check the following articles:
|Major Sites in Indus Valley Civilization||Indus Valley Civilization|
|100 Must-Know Facts about IVC for UPSC||UPSC FAQ|
|NCERT Notes: Arts of the Indus Valley||NCERT notes for History (Modern, Medieval, Ancient), Art and Culture, and Geography|