The Citadel is a large open space with temples surrounding it that was probably used for ceremonies. Walk across the square and climb the steps on the opposite side to view the Temple of Quetzalcoatl (the feathered serpent). The photo here shows the decoration on the facade of this building: alternating heads of the serpent and another figure, sometimes called Tlaloc (the Aztec rain god). The building is also decorated with snails and shells, symbols of water.
Next stop on your tour of Teotihuacan: the Avenue of the Dead.