This magnificent archaeological site planned and built as a fortress capital city after the fall of Teotihuacan was inhabited from 650 A.D. to 900 A.D. People of other regions gathered here, from Guerrero and Michoacan, valley of Oaxaca, Puebla-Tlaxcala, Gulf Coast, even the Maya, and there is an underground observatory used by priests and astronomers for religious purposes. In the Acropolis, where governing family lived, are apparent security rooms, accessible only from the roof. Elegant Pyramid of the Plumed Serpent indicates here Quetzalcoatl was transformed from minor deity to major presence in Mesoamerica. Declared a UNESCO World Heritage Site; there is a museum. Xochicalco is 30 minutes southeast of Cuernavaca in the countryside.