The structure stands 24 meters (79 feet) tall, with an extra 6 meters (20 feet) for the temple at the top. The square base is 55.3 m (181 ft) wide. There are more than 1000 blocks in the structure. El Castillo was built as a religious monument by the Aztecs. They called it "the sacred tower" and used it to watch over their empire. Today, it is one of Mexico's most important archaeological sites. El Castillo was built between 1487 and 1522. It started out as just one building but later on they added more and more floors. The final version had eight floors plus the ground floor which served as a temple. The project took about 10 years to complete. Many people died while working on the project so you can imagine how dangerous it must have been to build such a thing back then! Today, only parts of the original structure remain because over time elements were taken away to be used in other buildings or destroyed. In fact, most of what you see today is from the 19th century when the site was first visited by Europeans. At that time, many pieces were taken away by priests who wanted to sell them. Only now are we starting to find out where they went.