Basing on its date of occupation, Cuicuilco, now a significant archaeological site, may be the oldest city in the Valley of Mexico and was roughly contemporary with, and possibly interacting with the Olmec. It was the first important civic-religious centre of the Mexican Highlands and its population probably included all social strata and cultural traits of classical Mesoamerica.
The site facilities include terraces, various buildings, fortifications, irrigation ditches and canals. The main known structure is a pyramidal basement built about 800–600 BCE. Today, at the site of Cuicuilco are 8 of the many housing and religious buildings that once existed, and the remains of a hydraulic system that supplied water to the city. One of the pyramids was built in a strategic position, representing early pre-Hispanic attempts to link religious concepts with cosmic events through building construction.
Cuicuilco was originally founded as a farming village, but it also presents evidence of early religious practices, including stone offerings and the use of ceramics as grave goods. The city grew around a large ceremonial centre with pyramids and an associated urban area that included plazas and avenues bordering a series of small, shallow pools.
Population at the city’s peak is estimated at 20,000 people. The main known structure is a pyramidal basement built about 800–600 BCE. Archaeological evidence, ceramics and structures indicate that Cuicuilco developed during the first millennium BCE, during the pre-classical period, as a small settlement. Its inhabitants interacted with other sites, in the Mexico basin as well as relatively distant regions, such as Chupicuaro to the West and Monte Albán Southeast. Estimated Cuicuilco occupation periods may be, at best, considered tentative. The earliest occupation is estimated to have existed around 1200 BCE. During the period 1000–800 BCE conical structures with oval bases were built. The specialists call these sites regional capitals, considering the fact they had higher hierarchy and functioned as integration centres, eventually becoming larger regional capitals. If the great pyramid of Cuicuilco is an expression of this increment, then this level of development was reached between 800–600 BC, when it was built. If true, the proto-urban characteristics might have extended to the late Preclassical period, the Cuicuilco weakening between 100 BCE and 1 CE. By the time when development of Teotihuacán began, it became an important Classical period urban centre. Archaeologists conclude that Cuicuilco was a prominent site prior to the emergence of Teotihuacán as an urban centre, noting that the six small communities, which some archaeologists believe eventually combined to become Teotihuacán, were founded and showing evidence of modest growth during the time when Cuicuilco was building pyramids and public monuments. The city seems to have been abandoned around 150 to 200 AD, after the eruption of a nearby volcano, Xitle, although the territory was reoccupied at a much later date. Pottery and other evidence suggest that refugees from the volcanic disaster migrated north and became part of the population pool of Teotihuacán, near the northern shore of the Lake Texcoco.
The site of Cuicuilco is covered by a dense volcanic lava field. The lava covers an area of approximately 80 square kilometres, from the Ajusco mountain range to a nearby lake shore. A 1956 study concluded that the uneven lava deposits, reaching a depth over 10 metres in areas, were a major factor in the preservation of Cuicuilco.
The archaeological site is inside a modern urban area, and it is partially covered by buildings associated with the National University of Mexico. Only partial archaeological investigation has been possible, and modern building techniques have damaged the prehistoric city. Several 1990 archaeological sites at Cuicuilco (part of a circular pyramid constructed within a plaza with smaller structures associated with agricultural system) were destroyed for the construction of a multistorey office complex. Consequently, the true size and complexity of Cuicuilco may be difficult to ascertain.
From their location, inhabitants of Cuicuilco had access to natural resources (flora and fauna), as they were located approximately 4 kilometres from the Xochimilco Lake. Prehispanic groups managed to produce food. The economic Mesoamerican base was centred on agriculture, probably supplemented with hunting, fishing and gathering.
Access to wood had to be simple, from nearby forests, and agricultural lands in the vicinity of the nuclear portion of the site are buried today under metres of volcanic lava and modern buildings. It is believed that the diet of Cuicuilco inhabitants consisted mostly of corn, beans, squash, tomatoes, fish and wild animals.