In the study of history as an academic discipline, a primary source is an artefact, document, diary or any other piece of evidence that can be tied directly to the subject being studied.
In the case of Kota Gelanggi, this would be a letter written by an ancient merchant while he was actually in the city or if we were to find the city itself, that would be classified as a primary source itself.
Secondary sources are usually documents or recordings that discuss primary sources. They involve generalisation, interpretation, analysis or evaluation of primary sources. These sources could be written hundreds of years after the event and are usually not contemporaneous with it. In Kota Gelanggi's case, its mention in the 17th century Malay literary work Sejarah Melayu would be considered a secondary source.
This article would be considered a tertiary source as it is
In fact, the only evidence we have so far of the city's existence are from secondary sources, such as the aforementioned Sejarah Melayu, and the folk histories of the Orang Asli living in the area.
In fact throughout the years the Orang Asli living in the area have insisted the city did exist and have offered to take explorers there numerous times.
Sir Richard Olaf Winstedt, a colonial administrator, was reported to have said that the Orang Asli in the area were more than willing to take people to the site in the late 1920s.
In the present day, Orang Asli elders from the Lingui Dam area have confirmed its existence and even say they have seen the city first hand.
However, the area is apparently considered pantang (taboo) by the Orang Asli as they say it is full of vengeful spirits and tigers.