The Kalasasaya (kala for stone; saya or sayasta for standing up) or Stopped Stones is a major archaeological structure that is part of the UNESCO World Heritage Site of Tiwanaku. The Kalasasaya is low platform mound with a large courtyard that is surrounded by high stone walls. The Kalasasaya is about 120 by 130 meters in dimension and aligned to the cardinal directions. Like the other platform mounds within Tiwanaka, it has a central sunken court. This sunken court can be reached by a monumental staircase through an opening in its eastern wall. The walls are composed of sandstone pillars that alternated with sections of smaller blocks of Ashlar masonry and incorporates tenon heads of many different styles. This wall, as it currently stands, has been reconstructed in modern times. The Kalasasaya dates back to at least 200 BCE - 200 CE. It is located to the north of the Akapana and west of the Semi-Subterranean Temple.[1][2]
The Kalasasaya was used as a ceremonial center and for astronomical observations, allowing users to observe and define certain astronomical activities on any date of the 365-day year. On the Fall and Spring equinoxes (21 March and 21 September, respectively, for the southern hemisphere) the light of Sun shined through the main entrance gate. This indicates that the Tiwanaku civilization understood earth/sun cycles (calendar) and astronomy well enough to incorporate them into their construction projects and activities.