Remember reading that their cities were not fortified and were probably destroyed by waves of invaders - probably Indo-European Steppe nomads ancestral to the Persians and Sanskrit speaking Aryans.
Agree. But there are large numbers of Indian nationalists who insist that the Aryans are “native” to India, and that Indo-European languages spread out of India to the rest of Asia and Europe.
For some obscure reason they believe the “Aryan invasions” scenario is insulting to today’s Indians. I don’t get it, since Europeans aren’t insulted by claims that Indo-European speaking peoples similarly invaded Europe. Or at least western Europe.
The Indus basin also turned out to be the area from which Neolithic beads originated — the hand-drilled beads had been found throughout the Middle East and Central Asia, and study showed specific patterns of drilling inside the holes that apparently reveal individual workshops / craftsmen / locales. Probably settled life and urbanization had deep roots before, as you said, the IndoEuropean invasion. The cities ran things, didn’t need walls, a situation found in plenty of other places, including classical Greece, and to some extent, Mycenaean Greece; Rome had no city wall until Emperor Aurelian; Pharaonic Egyptian cities were unwalled. The places where walled cities were common were the very places that got invaded a lot, such as Canaan (e.g. Jericho).
Indo-European nomads (Aryans) theory has been discredited by DNA. Careful DNA research has indicated that the flow of people was south-to-north!