Based on old texts that mention what are very likely ancient ruins, and Gallo-Roman archaeological discoveries, we can say with certainty that the Butte Montmartre was occupied starting in Antiquity. The topography (a high spot, and the presence of a spring) as well as the toponym (Mont de Mercure or Mont de Mars) could lead one to identify it as a Gallic place of worship. However, there is no proof of a religious function during this period, although one did appear after the Roman conquest. This is attested to by the 19th century discovery of the foundations of a Roman temple.
The exact location of this temple has been lost, but it was probably situated near the Moulin de la Galette and was, according to local scholarly tradition, dedicated to Mars. Another temple dedicated to Mercury was located near Sacré-Coeur and the Montmartre church.
The existence of a bathing establishment equipped with a hypocaust, at the present-day Lamarck Caulincourt intersection, has been well documented. Its proximity to the hill's water source, whose water it used, leads one to think that this was a sanctuary dedicated to a local water god.
Church tradition places the martyrdom of Saint Denis-the supposed first bishop of Paris, but about whom we know nothing-on the Butte. This, and the transformation of the name Mont de Mars (or Mercure) into Mont des Martrys, could indicate the Church's desire to Christianise what had been a pagan place of worship.