The Crypte Archeologique (Archaeological Crypt) in Notre Dame Square (Parvis) in Paris is an incredible site for those interested in the history of Paris. During the Gallo-Roman Period, Paris was known as Lutetia, which developed from the first and second centuries BC.
The Crypte Archeologique contains the remains of Gallo-Roman Lutetia, including its third century BC walls, its streets and heating systems and even the ruins of a cathedral. Some of the remains are medieval, dating to the sixteenth to eighteenth centuries and including a hospital.
A little known site, the Crypte Archeologique is often not as crowded as the streets above it.