El Castillo, Chichén Itzá
One of the most famous ancient structures in the world, El Castillo (The Castle) is located in Chichen Itza, on the Yucatan peninsula. What you see was built about 1100AD over the top of a much older pyramid. El Castillo exhibits many elements of the elaborate Mayan calendar system. The solar calendar is represented by the stairs, which total 365. There are 9 tiers, which when split by the staircases yields 18, the number of months in a year. There are 52 decorative panels on each side, equalling the number of years in the calendar cycle. In addition, on the solstices and equinoxes, unique shadows were cast by the pyramid edges on the stairs, marking both these significant religious events and as an indicator it was time to plant.