Unless I am incorrect, the extent to which we can call those "permanent structures" is debatable. Most of those structures are now gone, and those that remain have become increasingly overgrown to the point that they resemble grassy hills. Not exactly Giza.
They have been purposefully flattened for agriculture and housing in the past, but to call them non permanent is rather odd for a structure that’s been standing for over a thousand years. And it was an urban area equivalent to the largest in contemporary Europe.
A mound made of earth rather than rock or metal is unusual for a structure, but that doesn’t make it “not a structure.” Many of our oldest cities appear on mounds unintentionally built over thousands of years as a new city was built on the old. This was a city that could survive extremely high floods; in fact, it was better suited for floods than virtually any of its contemporary cities in the Old World.
What’s more, the movement of such vast amounts of earth is very, very difficult, and it is a feat comparable to a smaller version of Giza. Unlike almost all pyramids, it functioned as a place to live, as an extension of how much land the city had. It was very similar to the tactics used in the Raising of Chicago, in that it increased drainage (very important for urban areas), protected from floods, and, unlike the latter, increased the area of livable land.
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