The most dramatic structure in Chichén Itzá is El Castillo (the Castle), also known as the Temple of Kukulcán. At 24 meters (79 feet) it’s the tallest structure on the site, and certainly the most recognizable. Dating to around A.D. 850, the Castillo was built according to strict astronomical guidelines.

There are nine levels, which, divided by the central staircase, make for 18 platforms, the number of months in the Maya calendar. Each of the four sides has 91 steps; together with the platform on top, there are 365 steps, or one for each day of the year. There are 52 inset panels on each face, equal to the number of years in each cycle of the Calendar Round.

On the spring and autumn equinoxes (March 21 and September 22), the afternoon sun lights up a bright zigzag strip on the outside wall of the north staircase and the giant serpent heads at the base, giving the appearance of a serpent slithering down the steps. Chichén Itzá is mobbed during those periods, especially by spiritual-minded folks seeking communion with the ancient Mayas. The effect also occurs in the days just before and after the equinox, and there are many fewer people blocking the view.

Climbing the Castillo used to be a given for any visit to Chichén Itzá, and the views from its top level are breathtaking. However, an elderly tourist died in 2005, after tumbling from near the top of the pyramid to the ground. The accident, combined with longtime warnings from archaeologists that the structure was being irreparably eroded by the hundreds of thousands of visitors who climbed it yearly, prompted officials to close it off. Pyramids at other sites have been restricted as well, and it’s looking more and more like a permanent policy.

Deep inside the Castillo, and accessed by way of a steep, narrow staircase are several chambers. Inside one archaeologists discovered a red-painted, jade-studded figure of a jaguar.