The clock tower is the other surviving fragment of the medieval Basilica of Saint Martin — the south-western tower of the lost church. It sits embedded in a block of later buildings on Rue des Halles, easily missed if you don't know what you're looking for.
The contrast with the Tour Charlemagne, two streets away, gives the best sense of the basilica's enormous size: the two towers stood at opposite corners of the nave, and the space between is now built over with shops and houses. The tower itself is closed to visitors but worth a glance from the street.