“Every city was surrounded by a wall, but once inside, residents would find themselves walking past several more walled enclosures. We’re not entirely sure why the Harappans designed their cities this way...”
Here is a hint: You build walls to keep things out (or in). You don’t invest in massive labor and construction costs just because walls look pretty. Walls around cities suggest...what? That they were worried about illegal immigration, or...attackers?
Could be non-human attackers (and i’m not talking zombies :) — I mean wild animals