Chartres Cathedral is a spectacular example of the way Christianity usurped the old pagan religions and literally built its churches over the sacred sites of the pagan world in an attempt to erase them from history.
According to Chartres Cathedral.net, "the town of Chartres has been of religious importance since ancient times. It was the site of Druid ceremonies, which were held around a well that was later discovered under the cathedral crypt.
"Sometime later, a Gallo-Roman temple stood on the same spot. The early Christians erected a basilica there during the 4th century, and St. Bernard preached the Second Crusade there in 1146. Later in history, Chartres was the coronation site of Henry IV in 1594."
Chartres is surrounded by legends, many of which can be traced back to pagan beliefs. According to Ancient Origins, "[the legendary roots of the site] come from the time when Druids, the Celtic priests of Britain and Gaul, held sacred rites there." A Druidic statue of the mother goddess of infants found at the site may have inspired the Christian legend that the pagans "had foretold the story of a virgin who would give birth to a child for the good of the world." This is speculative, but it makes sense considering the way the Christians hijacked pagan beliefs to support their new religion.
"During the Gallo-Roman times, the underground waters of the site were believed to have been revered by the Druids. Legend also says that Roman soldiers attacked Christians while they were celebrating their faith at the virgin shrine. It seems that Christians identified the mother goddess of the Druids with the virgin Mary." (Ibid)
The cathedral has been associated with the Knights Templar, the famous Catholic military order founded in 1199 to protect pilgrims traveling to the Kingdom of Jerusalem after its liberation by the First Crusade. Originally consisting of only nine men who could do little more than provide an armed escort for pilgrims to the Holy Land, the order grew into one of the richest and most powerful organizations of its day until it was violently suppressed and virtually exterminated in 1307.
Surrounded by conspiracy theories then as now, "[t]he Knights Templar have become associated with legends concerning secrets and mysteries handed down to the select from ancient times," (Wikipedia). According to Ancient Origins, the Chartres cathedral's association with the Templars comes from the two black madonnas which can be seen there:
"According to legend, the Black Madonna was a lady of the Templars. The statue of the Black Madonna with a child was commissioned in 1508. It is perhaps a black wooden copy of the silver Madonna that was located on the main altar before. The statue sits on the top of the pillar and is often called the ''Lady of the Pillar''. Before the French Revolution, the Chartres Cathedral had a second Black Madonna in the crypt. Unfortunately, it was probably burned during the revolution, although some resources are convinced that the church didn't suffer in this period. Today, tourists and pilgrims can see a copy in the church that was made in 1857. "
These black madonnas are apparently found in churches all over France and other countries in Europe.
"Throughout Europe, especially in France, about 500 Black Madonnas, painted or carved in wood or stone, stand in Catholic churches, with a few in museums. Most date from the 11th to the 15th centuries. These medieval images of the Virgin holding the Christ Child gaze benignly on their devotees, who come as pilgrims and associate them with miracles and healing." Source: Europe Up Close.
I'm not sure what to think about the Templar connection with these images, but they seem to have a pre-Christian origin. According to Europe Up Close, "[m]any scholars say that the Black Madonna is a link to religions that existed long before Christianity. Her shrines are often in locations that were once pagan shrines to Diana, Artemis of Ephesus, Cybele and other goddesses. Some Black Madonna figures look just like the Egyptian goddess Isis with her infant son Horus, showing the influence of old ways on changing faiths. Even further back than that, she may represent the archetypal Earth Mother, the ancient, eternal power of the feminine force."
The great Oswald Spengler saw the Gothic cathedrals as prime symbols of the Faustian spirit of Western civilization, but even though Spengler believed that the Classical and Western worlds were fundamentally separate, the pre-Christian influences on Christianity, especially on Catholicism, can be seen both in its teachings and its architecture. These fantastic cathedrals contain all sorts of mysteries that can be traced back to very ancient times. As for the Chartres cathedral, it's currently being restored (I'm not sure if the work has been completed yet). The renovation has apparently created a lot of controversy.
Video from 2016.