The ancient Sumerians were a very religious people. They believed that everything that happened good or bad was a result of their gods. They worked hard to make their gods happy. This was quite difficult since their gods, and they had hundreds of gods, were not a happy bunch. In fact they were downright grumpy. So the Sumerians spent a lot of their time and effort seeking new ways to please their gods.
The ancient Babylonians were a lot like the Sumerians. The Babylonians were the invaders who overthrew the Sumerian city-states in the south and took over the region. They adopted the Sumerian culture, government, economy, urban living, writing (cuneiform), law, and religion. They had the same bunch of gods, with some more added to them. The Babylonians differed from the Sumerians in that their gods had a hierarchy. Marduk, who to the Sumerians was just another god, was the leader of the gods to the Babylonians. The other gods worked for Marduk with some being much more important than others. Like the Sumerians, the Babylonians spent time and effort in trying to make their gods happy. The city of Babylon was built to honor Marduk.
The Assyrians worshiped most of the same gods as Sumerians. Like the Babylonians, they added more gods. They recognized Marduk as an important god, but he was not the most important. To the Assyrians, the most important god was the god Ashur. They built buildings in their towns and decorated them with dragons and demons to protect themselves from the gods.