@Joe Shearer
I am a patient man. No need to rush.
Who were they.
What happened to them.
Did some (or most) survive whatever put a full stop to their civilization.
Where did these go.
They probably never needed a defense because no hostile force knew they were there.
When they did (need defence) they were clearly unprepared and I'll equipped.
I believe they were driven into the sanctuary of India's dense forests.
The Gangetic plains were still forested. And not open farmland like today.
I don't know whether they were absorbed (genetically and eventually culturally) into the jungle tribes.
Whether they became foragers and gatherers if not hunters.
Or whether they got pushed from the east towards the south.