This video explains that much history of the Incas is not known due to them not having a written language, with only a few post-Columbian accounts being written down that were based their oral history at that time.  It explains that the city of Cuzco was initially built by the Incas, with the Spanish often simply just adding to the already existing building foundations.  The Inca empire spanned 2,500 miles along the Pacific coast from present-day Columbia to Chile, where it ruled as many as 12 million people.  The video explains how the Spanish had an agenda of erasing Inca culture, which further obscured their history.  Several mixed-race Incas such as Blas Valera and Inca Garcilaso de la Vega wrote accounts at the time based on their oral history.