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That’s about the density of Kowloon walled city. And ignores the available space in the ocean.

Did they grow any food, produce electricity, or had any manufacturing at all in Kowloon?

To be clear, "Kowloon" is different from "Kowloon Walled City". And, yes, they did have very small manufacturing inside.

I highly recommend that you find a copy of this (in)famous book about it: << City of Darkness>> (https://cityofdarkness.co.uk).