As one can imagine, in such a vast, dry, and flat terrain, there was not much in the choice of available building materials. One thing there was plenty of, though, was mud. And the Sumerians knew exactly how to build with mud, making sun-baked bricks for the foundation and center, and using fire-baked bricks for the rest. They would start with a foundation and then build the various tiers, each tier built on the platform roof of the tier underneath it. The higher that they could build, the better it was seen, as ziggurat buildings were meant to act as a bridge between this world and the heavenly realm.