As the source itself acknowledges there are problems in defining a city, and claims based on historical records are contentious. Neverthess, according to the source only one city in Southeast Asia holds the title of once being the most populous in the world.

I will leave this post without a picture or tags for now as the architecture and religious iconography is distinctive, but if you would like a hint there is one here.

When you are ready the answer is in the link here.