El Castillo, Chichén Itzá

A calendar in stone

El Castillo is a clock as much as a temple. Each of its four stairways has 91 steps; four times 91 is 364, and the platform at the top makes 365 — the days of the solar year. The nine terraces, split by each stairway, read as eighteen, the number of months in the Maya calendar.

It was built to Kukulcán, the feathered serpent, the Yucatec Maya form of the god the Aztecs called Quetzalcoatl.

The serpent of light

Twice a year, the building performs.

Around the spring and autumn equinoxes, the late-afternoon sun catches the stepped northwest corner and throws a row of triangular shadows along the edge of the northern balustrade. Joined to the carved serpent's head at the foot of the stair, the shadows form a snake of light and dark that appears to ripple down the pyramid as the sun drops. It is a deliberate solar alignment, and it still draws crowds to the site on the equinox today.

A city of wonders

El Castillo is only the centrepiece. The Great Ball Court beside it is the largest in the Americas, its walls carrying carvings of the deadly Mesoamerican ball game and a famous echo.

The round tower called El Caracol was an observatory, its windows aligned to the movements of Venus. And a short walk north, a vast natural sinkhole — the Sacred Cenote — received offerings of jade, gold and, the dredged remains show, people. In 2007 Chichén Itzá was named one of the New Seven Wonders of the World.