"El Castillo" (the castle) - main building
. This step pyramid has a ground plan of square terraces with stairways up each of the 4 sides to the temple on top. On the Spring and Autumn equinox, at the rising and setting of the sun, the corner of the structure casts a shadow in the shape of a plumed serpent - Kukulcan, or Quetzalcoatl - along the side of the North staircase. On these two days, the shadows from the corner tiers slither down the northern side of the pyramid with the sun's movement.