In the 18th century, the tower of the Hôtel de Cluny was used as an observatory by the astronomer Charles Messier who, in 1771, published his observations in the landmark Messier catalog.
The Hôtel de Cluny is partially constructed on the remnants of the third century Gallo-Roman baths (known as the Thermes de Cluny), famous in their own right, and which may be visited.