Tenochtitlan
The city had a huge pyramid called the Templo Mayor (Great Temple).  At the top of the Temple, there were two small rooms.  In these rooms, the Aztecs sacrificed people to two of their most important gods: Huitzilopochtli, the god of war and the sun, and Tlaloc, the rain god.  The Aztecs believed that if they did not feed their gods blood, the gods would wither and die, causing the end of the world as they knew it.