Towering above the other buildings at 79 feet (24 m) high, the Pyramid of Kukulkán called "El Castillo" (the castle). Two of its sides have been completely restored, the other two were left to show the condition before work commenced. Each side had originally 91 steps, adding the platform at the top as a final step there are 365 in total one for every day of the year. Further evidence that this building was linked to the Mayan interests of astronomy and the calendar is demonstrated at the spring and autumn equinox. On these days the shadow of the sun playing on the stairs causes the illusion of a snake processing down the pyramid in the direction of the cenote.

If you stand facing the foot of the temple and shout the echo comes back as a piercing shriek. Also, a person standing on the top step can speak in a normal voice and be heard by those at ground level for some distance. This quality is also shared by another Mayan pyramid at Tikal.