In ancient Mesopotamia, ziggurats were very tall temples where Sumerians could practice their religion. According to them, they built these ziggurats so tall so that they could reach the heavens and communicate with the gods. Sumerians were a polytheistic nation, which means that they believed in more than one god, including Enlil, Enki, Nergal, etc. So polytheism and ziggurats are connected because ziggurats are a way for people to reach the gods they believed in.