When Teotihuacán began to weaken around 650 to 700 CE, during what is called the Epiclassic period, Xochicalco began to rise in importance, achieving its peak between 650 and 900 CE, with far-reaching cultural and commercial relations. Xochicalco remained an important center until around 1200, when its excessive growth precipitated a demise similar to that of Teotihuacán.

Xochicalco is the supreme example of this type of Epiclassic fortified city and the data indicate that the was a well-planned urban center. In addition, it has been speculated that it may have had a community of artists from various parts of Mesoamerica for the extraordinary works that have been reflected in the buildings.

Substantial engineering work, in the form of terracing and massive retaining walls, creates a series of open spaces that are defined by platforms and pyramidal structures. These are linked by a complex system of staircases, terraces and ramps to create a main north-south communication axis. Additionally, the builders achieved to give the hill itself the shape of a pyramid.

The most famous monument is the “Pirámide de Quetzalcóatl” (Pyramid of the Plumed Serpents). The four facades are sculpted in high relief with representations of enormous plumed serpents, the Quetzalcoatl and the upper walls of the temple bear figures that have been interpreted as warriors.

Another feature of the site is the presence of caves in the hillsides which were used for storage and, in in particular one was used as an underground observatory. This latter has a man-made shaft to the sky and was known as “El Observatorio” (observatory). The chimney can be used to observe the sun’s journey to the Tropic of Cancer on May 14 or 15, and its return journey on July 28 or 29 annually.

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