Like the Egyptians at the same time, the Sumerians around 3000-2500 BC devoted a lot of energy to building big buildings. But unlike the Pyramids, which are tombs for dead Pharaohs, the Sumerian ziggurats (ZIG-oo-rats) are temples for their gods.
Because good building stone is hard to find in the river valley of the Euphrates River where the Sumerians lived, the Sumerians mostly did not build in stone. Instead, they built their ziggurats (and also their houses and city walls) out of mud-brick, or adobe.
Ziggurats are very high buildings. You start by making a big flat platform
of mud-brick, and then you make a slightly smaller platform on top of
the first one, and another on top of that, until the platform is just
a little bigger than a temple, and then you build the temple at the
very top, rather like a sand-castle. Maybe they thought it was better
to pray to the gods from as close as possible, and so if the gods lived
up in the sky you had to build great platforms to get near them.
Of course it isn't very hard to build a very impressive building this way: it is solid all the way through, so it is easy to get it to stay up.
To find out more about ziggurats, check out these books from Amazon.com or from your library:
The Sumerians, by Elaine Landau (1997). For kids. Despite the bad Amazon rating, this is a good solid introduction to the Sumerians, with an explanation of prehistory at the beginning for context. Pictures of ancient stuff, and good maps.
The Art and Architecture of the Ancient Orient, by Henri Frankfort (5th edition 1997). The standard for college art history classes.