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 west side of the north staircase. On these two annual occasions, the shadows from the corner tiers slither down the northern side of the pyramid with the sun's movement to the serpent's head at the base.

Mesoamerican cultures periodically built larger pyramids atop older ones, and this is one such example. In the mid 1930s, the Mexican government sponsored an excavation of El Castillo. After several false starts, they discovered a staircase under the north side of the pyramid.