"At the base of the stairs of El Castillo are 2 large serpent heads.
In Mayan religion, 2 serpents expressed the idea of "Precious Twins", the Sun & Moon, or maybe the two solstices.

The open mouths of the rattlers symbolize the act of Creation, for the Sun-god was conceived in the form of a rattlesnake."
..Jose Diaz-Bolio from "Guide to the Ruins of Chichen Itza"