From my understanding it was a pagan sun God site where rituals were performed during solstices and other dates relative to the sun. The book wasn't specific as to the nature of the rituals. It was later used as the site for other pagan religious gatherings. That's why Vatican City was built there. Because it was already a known place. It denied pagans their own site and offered a new religious ritual under the guise of Christianity.

i heard its built over a portal

putain d’eglise romaîne 🙄